Other physical (samatha) meditation objects besides breath? Does Buddhist samatha meditation allow for physical meditation objects other than the breath? I have noticed I feel much more contentment with the physical sensation of "back straight". Please give examples or references.

Ayya Khema writes (Know Where You're Going) :


  In the beginning we need the necessary meditation practice, the methods that will be our key to open the door, but within them we must find something that is of particular help to us. That element can be physical or mental, concerned with posture or with thoughts.


Upasika Kee writes (Pure and Simple):


  Sit straight and place your hands one on top of the other on your lap. Hold your head up straight and keep your back straight—as if you had a yardstick running along your spine.
yes, when it comes to rupa, there is bhāvana on ''rupa'', or better said, bhāvana  on each one of the five properties
http://obo.genaud.net/dhamma-vinaya/ati/mn/mn.062.than.ati.htm#p1


  "How, lord, is mindfulness of in-and-out breathing to be developed and
  pursued so as to be of great fruit, or great benefit?"
  
  "Rahula, {any form whatsoever that is past, future, or present;
  internal or external; blatant or subtle; common or sublime; far or
  near: every form is to be seen as it actually is with right
  discernment as: 'This is not mine. This is not my self. This is not
  what I am.'
  
  There are these five properties, Rahula. Which five? The earth
  property, the water property, the fire property, the wind property,
  and the space property.
  
  "And what is the earth property? The earth property can be either
  internal or external. What is the internal earth property?}[3]
  Anything internal, within oneself, that's hard, solid, and sustained
  [by craving]: head hairs, body hairs, nails, teeth, skin, flesh,
  tendons, bones, bone marrow, kidneys, heart, liver, membranes, spleen,
  lungs, large intestines, small intestines, contents of the stomach,
  feces, or anything else internal, within oneself, that's hard, solid,
  and sustained: This is called the internal earth property. Now both
  the internal earth property and the external earth property are simply
  earth property. And that should be seen as it actually is present with
  right discernment: 'This is not mine, this is not me, this is not my
  self.' When one sees it thus as it actually is present with right
  discernment, one becomes disenchanted with the earth property and
  makes the earth property fade from the mind.
  
  "And what is the water property? The water property may be either
  internal or external. What is the internal water property? Anything
  internal, belonging to oneself, that's water, watery, and sustained:
  bile, phlegm, pus, blood, sweat, fat, tears, oil, saliva, mucus,
  oil-of-the-joints, urine, or anything else internal, within oneself,
  that's water, watery, and sustained: This is called the internal water
  property. Now both the internal water property and the external water
  property are simply water property. And that should be seen as it
  actually is present with right discernment: 'This is not mine, this is
  not me, this is not my self.' When one sees it thus as it actually is
  present with right discernment, one becomes disenchanted with the
  water property and makes the water property fade from the mind.
  
  "And what is the fire property? The fire property may be either
  internal or external. What is the internal fire property? Anything
  internal, belonging to oneself, that's fire, fiery, and sustained:
  that by which [the body] is warmed, aged, and consumed with fever; and
  that by which what is eaten, drunk, chewed, and savored gets properly
  digested; or anything else internal, within oneself, that's fire,
  fiery, and sustained: This is called the internal fire property. Now
  both the internal fire property and the external fire property are
  simply fire property. And that should be seen as it actually is
  present with right discernment: 'This is not mine, this is not me,
  this is not my self.' When one sees it thus as it actually is present
  with right discernment, one becomes disenchanted with the fire
  property and makes the fire property fade from the mind.
  
  "And what is the wind property? The wind property may be either
  internal or external. What is the internal wind property? Anything
  internal, belonging to oneself, that's wind, windy, and sustained:
  up-going winds, down-going winds, winds in the stomach, winds in the
  intestines, winds that course through the body, in-and-out breathing,
  or anything else internal, within oneself, that's wind, windy, and
  sustained: This is called the internal wind property. Now both the
  internal wind property and the external wind property are simply wind
  property. And that should be seen as it actually is present with right
  discernment: 'This is not mine, this is not me, this is not my self.'
  When one sees it thus as it actually is present with right
  discernment, one becomes disenchanted with the wind property and makes
  the wind property fade from the mind.
  
  "And what is the space property? The space property may be either
  internal or external. What is the internal space property? Anything
  internal, belonging to oneself, that's space, spatial, and sustained:
  the holes of the ears, the nostrils, the mouth, the [passage] whereby
  what is eaten, drunk, consumed, and tasted gets swallowed, and where
  it collects, and whereby it is excreted from below, or anything else
  internal, within oneself, that's space, spatial, and sustained: This
  is called the internal space property. Now both the internal space
  property and the external space property are simply space property.
  And that should be seen as it actually is present with right
  discernment: 'This is not mine, this is not me, this is not my self.'
  When one sees it thus as it actually is present with right
  discernment, one becomes disenchanted with the space property and
  makes the space property fade from the mind.
  
  "Rahula, develop the meditation in tune with earth. For when you are
  developing the meditation in tune with earth, agreeable and
  disagreeable sensory impressions that have arisen will not stay in
  charge of your mind. Just as when people throw what is clean or
  unclean on the earth — feces, urine, saliva, pus, or blood — the earth
  is not horrified, humiliated, or disgusted by it; in the same way,
  when you are developing the meditation in tune with earth, agreeable
  and disagreeable sensory impressions that have arisen will not stay in
  charge of your mind.
  
  "Develop the meditation in tune with water. For when you are
  developing the meditation in tune with water, agreeable and
  disagreeable sensory impressions that have arisen will not stay in
  charge of your mind. Just as when people wash what is clean or unclean
  in water — feces, urine, saliva, pus, or blood — the water is not
  horrified, humiliated, or disgusted by it; in the same way, when you
  are developing the meditation in tune with water, agreeable and
  disagreeable sensory impressions that have arisen will not stay in
  charge of your mind.
  
  "Develop the meditation in tune with fire. For when you are developing
  the meditation in tune with fire, agreeable and disagreeable sensory
  impressions that have arisen will not stay in charge of your mind.
  Just as when fire burns what is clean or unclean — feces, urine,
  saliva, pus, or blood — it is not horrified, humiliated, or disgusted
  by it; in the same way, when you are developing the meditation in tune
  with fire, agreeable and disagreeable sensory impressions that have
  arisen will not stay in charge of your mind.
  
  "Develop the meditation in tune with wind. For when you are developing
  the meditation in tune with wind, agreeable and disagreeable sensory
  impressions that have arisen will not stay in charge of your mind.
  Just as when wind blows what is clean or unclean — feces, urine,
  saliva, pus, or blood — it is not horrified, humiliated, or disgusted
  by it; in the same way, when you are developing the meditation in tune
  with wind, agreeable and disagreeable sensory impressions that have
  arisen will not stay in charge of your mind.
  
  "Develop the meditation in tune with space. For when you are
  developing the meditation in tune with space, agreeable and
  disagreeable sensory impressions that have arisen will not stay in
  charge of your mind. Just as space is not established anywhere, in the
  same way, when you are developing the meditation in tune with space,
  agreeable and disagreeable sensory impressions that have arisen will
  not stay in charge of your mind.