Question: Given an Earth-like planet, what sort of geological features or weather systems are required for a place to have the same average day and night temperatures throughout the whole year? Is such a thing even possible?

The closest I could think of were tropical climates that are hot all year long (though even these have cooler wet seasons), but I'm looking for something more temperate, around 15-25°C during the day.

The minimum area of the region should be roughly as big as Belgium.

Answer: 
Seasons are pretty universal, being caused by the angle at which the sun's light hits the planet. To make a point on a planet without seasons, you have two main options:

1. Give the planet's axis no tilt. This will allow you to have different regions with different temperatures depending on their distance to the equator, but no seasons anywhere else on the planet either.
2. Put the planet further away from the sun, so it is cooler overall. The tropics will still be season-less just like on Earth, but their temperature will be cooler.

A planet with an extremely thick atmosphere that blocks out the sun and heats the planet through greenhouse gases will have less temperature variation over the entire planet. Of course, such a planet will not actually be Earth-like.
A large body of water is **part** of the answer.

The water circulation in the Pacific Basin is clockwise (bringing cold water from Alaska to California) in the summer, and *counter*-clockwise (bringing warm water from the tropics to California) in the winter. The average water temperature off San Francisco (and probably also San Diego) is *higher* in the winter than in the summer. 

This has a lot to do with the nearly constant temperature in San Diego.