Standard 5g list:
1 Dwarf Puffer (Carinotetraodon travancoricus; no, I'm not being snooty, 'dwarf puffer' is applied to lots of puffers in my area, and Carinotetraodon travancoricus is called far more than DP in my area as well).
1 Betta (male or female, most prefer males), 2 African Dwarf frogs.
1 Betta, 1 Apple Snail (apple snails, with a bit of give, need about 2g per adult snail, depending on the type and your filtration and yadda yadda).
1 Betta (so many colors, tail types, species!)
1-3 Endlers (http

/swampriveraquatics.com/ if you're in the USA)
I've heard of a single guppy being recommended, but I've also heard that guppies generally prefer 10g+ and/or 20g+, and or 1g+... so who who knows what's what, that'd probably requrie further research.
Snailariums or Shrimp habitats have always been an idea that charmed me, personally. I think shrimps are nifty, they're so active and so interesting. I've also found the idea of planting and cultivating a 5g very charming, and then simply introducing a single apple snail (brig., the species that prefers to eat softer/nonalive plants) to the mix.
A few other people suggested 3 White Cloud Mountain Minnows, and then the suggestion as immediately discredited by others who believe that WCMMs have big wander-lust like Danios, and need large tanks to be happy.
Either way, the more complicated your inhabitants, the more complicated the care.
I have 1 male betta and 3 ADFs in one of my 5.5g's, and 1 male betta with an apple snail pending for a 5.3g. I like simple, low care tanks, because I realize my attention span can sometimes be greatly hindered by... shiney metal objects.
