I have a very very small aquarium (2L of water). What kind of animal could live in this size of aquarium with enough happinness? Maybe a starfish? Or some kind of crustacean?
Frankly, nothing will be happy in such a small tank.
I don't have experience with starfish and crusacean, but a quick google search indicates that they require bigger tanks.
If you're interested in setting up an aquarium with animals, then you'll probably need to have around 20L. Then you can start with shrimps or a single betta
I absolutely agree with Diether. You'll want something much bigger than that.
One additional thing to consider is also how difficult it is to keep a small aquarium within optimal operating parameters. The smallest change to the environment can have a big impact on your tank. For example, a fluctuation in temperature can change the water temperature in your tank hugely when there is only a small volume of water1.
A similar affect applies to pH, GH, KH, NH3 and NO3- during water changes. If the water you add back in has a different pH to your tank you could end up with a pH swing which has the potential to harm your fish.
Ultimately, it comes down to the life you want to provide your fish. If you're serious then do your research, take your time and seek advice on how best to proceed. Keeping fish can be enormously rewarding but it can be very cruel if you don't know the basics.