A lot of the options listed here seem to have little practicality.
Dwarf puffers would shred otos and the otos would starve without proper algal growth, mollies, platies, danios/white clouds, and guppies enjoy swimming too much to live happily in 5-gallons, and shrimp and bettas only mix well on the occasion because either the bettas harass, overstress, and kill the shrimps or the shrimp end up eating the tails on the bettas. Shrimp are also very delicate to water conditions and 5-gallons is not a very forgiving living space as far as perameter fluctuations go. Tiger barbs grow too large and need too much swimming room for a fiver.
I'm not sure how much experience you have with any of these species or ideas, so I'll lay them out like I would to a novice. I'm sure you aren't, but I want to be sure I cover anything you might not already know and have experience with, okay? ^.^;
Based on my own personal experiences with my 5-gallon mini-bow set up, I offer this idea. I have an 11-watt fluorescent bulb for my plants, plain gravel, and keep the pH around 6.8-7.0, temp 78 degrees. I also add liquid fertilizers with every water change (all items bought at Wal-Mart or PetCo)
My suggestion: java ferns, hygrophilia, 1 honey or dwarf gourami, 4 neon tetras or microrasboras, and a brigs apple snail.
My reasoning: Java and hygro can grow in just about any environment and are good for the overall health of the fish. They can be mixed with rocks, driftwood, and other artificial plants to get the desired effect. I'll gladly mail you some of my hygrophilia to get you started if you like.

I'd offer some java, but mine haven't started propagating just yet. The basic plant maintenance involved is that the hygro will need to be trimmed every couple of weeks. The stuff grows like a weed!!
Honey/Dwarfs are small and pretty and personable little fishes. They are also peaceful enough that a smaller tank won't cause them to become aggressive toward their tankmates 90% of the time. They are labrinth fish like bettas, but aren't as territorial and don't have the long drooping finnage some small fish like to nip at.
I have a Thai-imported betta in my tank, not a gourami, but he's fiesty enough that I'm not worried about fin damage from the neons... but he has been harassing them a lot, hence why I'm not suggesting a betta instead.
Tetras or microrasboras are fairly small and love well-planted environments. I've spoken with some here who have 5-gallon planted tanks with 6-8 of these little guys as a single-species tank. But to be on the safe side and give everyone some breathing room, go with four neons or microrasboras. MRs are expensive though and have to be shipped so neons or cardinals would probably be more cost-efficient. Initially, they are delicate and die easily (mostly due to shipping and poor care at the LFS), but once established will be content in a 5-gallon.
Apple snails are great for bottom-feeders in small tanks. Specifically brigs apple, as they only get to be about golfball sized and won't eat your plants unless they are
starving. They are great at consuming algae and excess fish food without taking up a lot of bio-load and provide something 'different' to look at. Some of the maintenance involved will be providing calcium and minerals for the snail to develop proper shell growth and the occasional veggie treat if there's not much algae to be had. I've thoroughly enjoyed my experiences with my snails.
You can try ramshorn snails instead if you like, but my two ended up breeding and covering the insides of two of my 3 aquariums with baby snails. If you want an apple snail, I can connect you with a fabulous breeder who supplied me with my snails. She sexed them for me so I didn't end up with unwanted apple babies and she has lovely colors. I have one blue, one jade, and one striped purple!
So there. A practical option based on personal experience. Even if you don't take my advice, the offers for the snail breeder and the excess hygro still stand. Good luck and make sure and post photos of the finished product!!!
