I'd go with a small Penguin or Cascade filter. More expensive, but the quality of filtration, the warranties, and the longer-lasting parts make up for it.
Fine grade substrate... perhaps a very coarse grade sand or a fine grade gravel. If you go for real plants, I'd suggest trying something like the dwarf (petite) anubias, since it's small and slow-growing and doesn't mind low light conditions.
http://cgi.ebay.com/Live-Tropical-A...ryZ66794QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I'd go with Nano Fishes... they are super tiny (most never growing to be a full inch long), so I bet you can have a school of 4 in a 5 1/2 gallon, no problem. I'd also get a trio of shrimp for bottom cleaners. Most FW shrimp also eat algae, so that'll help.
http://www.franksaquarium.com/nanofish.htm
Keep in mind, though, that if you go with nano fishes, they require a very fine-grade fish food for their super tiny mouths. Also, a color-enhancing or protein-rich food will help keep their colors nice and bright.
Yeah... I'd go with nanos... or I'd go with the aforementioned betta by itself in the tank. Depends on whether you want a lot of little fish or one big one.