I know what you mean about guppies and platties etc being boring... they just have no personality.
I am not suggesting all the following be mixed together, but trying to give some suggestions that go beying the usual cory, tetra, gourami suggestions. There are loads of great fish out there that nobody ever mentions.
Pearl gouramis are peaceful and beautiful fish, and not too big. It is great to watch them swimming around feeling all the other fish up. If i hold my hand aginst the glass of my tank they come up and try to feel it. Cute
Pencil fish are a lot nicer than tetras, in my opinion. And you get some with great colours.
Kuhli loaches are lovely little things. Most loaches in my experience have plenty of character, and are good fun to watch. Some, such as the clown loach will probably get too big for your tank though. dwarf chain loaches, and zebra loaches are two other good choices.
If you want character and colour together, you could always go for apistogramma cacatuoides or some of the other dwarf species.
Another interesting fish is badis badis. I have no experience of them though, so not sure what caring for them is like.
A few upside down catfish could be fun.... I would love some, but don't really have room for them right now.
I love my whiptail catfish. He does not do much, but is very pretty.
You could get and ancistrus for dealing with your algae.
Hatchet fish, although apparently sensitive (mine are hardy as hell), have a really great shape. Silver apparently more active than the marbled variety.
Threadfin rainbow fish are hardy, busy little things and in my opinion absolutely gorgeous, with a highly unusual shape. There are other rainbows as well which should be fine for your size tank. Do a search on them.
Hope that gives you more ideas on what is out there.
Oh yeah, keyhole cichlids are brilliant little fish which are very peaceful. I love the way mine get all excited when they see me walk in with the fish food!