What bag should I buy first for my gravel-touring kind of bike? I have bought a new bike, and I would like to start equipping it with bags, as I plan to do bicycle tours in the future. At the moment, however, I only do day tours, and I would like to buy a bag that would also serve me as a day pack. Basically, I would like it to be big enough to fit a couple of pieces of clothes, some snacks, my phone, and whatever else I might need for the day. I would like to have an easy access to all that stuff as well. What type of bags should I be looking at? 

I was thinking about a handlebar bag, and was having a look at those. But I seem to come across three types of bags: 1. roll up bags with no easy access to contents 2. accessory bags that seem too small for my needs 3. large handlebar bags that seem to be more suited for city bikes. Is there a handle bar bag that could suite my needs? Or should I be looking at other types of bags?

My bike is a kind of gravel/touring type. It does not have a rack, but I think it is equipped with all the possible mounting points.

In case it makes a difference, here is a picture of it:
For one day you are often fine with your jersey pockets and a small saddle bag. If needing more I would: 


increase the saddle bag


and/or 


add another small bag above the top tube behind your stem.



Photo Topeak

I would leave large saddlebags, handlebar bags, frame triangle bags and so on for serious touring. They can be awkward but have their place when you have to carry a lot of stuff. 

The next bag for me to add would be a frame triangle bag, but only when needed. It is still stable, fixed in the frame and does not move.


Photo Bikerumor

I see a lot of handlebar bags carried  by tourists on their hybrid bikes that are popular here, but for me they spoil the handling characteristics of the bike, you have to fight the bag when turning and they can move during the ride and so on. For a more serious rider I would add them when you need more stuff, not for a day ride.

The frame triangle bags can cause interference with your legs, the friction may damage your lycra shorts (if prolonged), so I would also add them later when needing more space. 

Although gravel bikes often do come with mounting points for racks/panniers, those are unpopular in modern bike-packing solutions where the preference is on carrying everything in the axis of the bike (not on sides) and the rear bag is mounted on the seat post.