Well with a 10 gallon tank you are fairly limited but there are lots of cool ones.
Easy to care for:
- Guppies: Very tolerant, but very messy. A good stock would be 5 of them in a tank, with maybe a centerpiece fish. You need around 1 male to 2 or 3 females, or all males.
Intermediate care:
- Celestial Pearl Danio: Newly discovered, fairly rare fish, but it's really cool! Pretty hardy and active, but quite shy, so make sure you have lots of hiding places. It can cope in pretty much any pH and hardness, so that isn't a problem. The best stocking for it would be 12 of them in a 10 gallon tank. They like lots of plants, and tons of cover. They don't do well with tankmates that are bigger, or a whole lot more boisterous (let alone agressive) then them. Due to their color, a dark substrate would be best.
- Chili rasbora: Similar to the CPD in that it is colorful, and thus a dark substrate would be best, and that they do well in most parameters. However, they are not as shy, and are far smaller, so a setup of 16 of them in a 10 gallon would work fine. They will do well with other fish IMO, but not anything very big.
- Honey gourami: Solitary fish, that is very capable of being kept with the other schooling fish listed here. Not so sure about the parameters, but it is quite hardy and very docile.
Advanced care:
- Microdevario kubotai: It's a cool new, very rare fish that can probably be treated as a CPD, because it is also pretty shy IMO and around the same size. I don't think it is a good beginners choice at all because it requires stable water conditions.
- Pencilfish: There are some types of pencilfish that can live in this tank, but they also need some live food I think. Probably not a good beginner's fish, but I barely know anything about them.
I personally recommend a tank like this:
- 1 honey gourami
- 8 celestial pearl danios
- 10 chili rasboras
It would be very well stocked, but it would be a cool tank.