Dwarf Gouramis are always a pleasant alternative if you decide not to go the Betta route (I agree with Barliman on his point about their potential aggression. Bettas tend to be intelligent/bombastic personality fish. Not one fish is going to be like the other). This is because dwarf gouramis will fill a similar role in providing colour and shape variety to the tank. These fish are going to be all over the tank, but they tend to be a little more shy than a Betta would be.
I have never had Apple Snails, but I have heard plenty of great things about them from the others on the forum. I think these guys will travel all over the tank. Someone with more experience with them will probably jump in at some point and better explain things. I have seen some recently that really caught my eye, so colourful and big. I've been tempted to get a 2 gallon fish bowl just to keep one while I'm at University.
I might have Multi-Tank Syndrome but I am fighting it as hard as I can.
Swordtails are, IMO, one of the more attractive livebearers around. Though this opinion is more that I really like having more unique body shapes in the tank, as well as a variety of different colour morphs. These guys tend to be all over the tank as well.
Oto catfish are another potential, a schooling fish too, though these guys are more sensitive than any of the aforementioned guys. Otos are a shy fish that tend to hang out in the bottom of the tank, or inside/around any decorations in the tank. You'd need to get sinking algae wafers for them and ensure that they are eating them. Not the best option for a tank that hasn't matured yet but I figured I would suggest 'em anyway.
Cardinal Tetras are amazing, for a dash of colour and schooling. They are mid-top tank fish and do best in groups of +7. I think a school of ten or more makes a tank look really nice, and they'll be providing the tank with more motion than the others would on their own, from my own past experience.
