Check craigslist and Freecycle if you want another 5-gallon or 10-gallon, you can get them for free or cheap
. I got a 50-gallon for free, in fact!
I also forgot, I wanted to suggest some alternatives to the frogs.
Besides the betta, some other ideas would be...
-A group of cherry shrimp. These guys are really neat and easy to care for. They do great in smaller tanks.
-A single pygmy gourami species (not the usual dwarf guys you see in stores but the tinier ones)
-A few amano shrimp (I kept 3 in a 5-gallon with live plants)
-A small group of white cloud minnows, cherry barbs, harlequin rasboras, or celestial danios with live plants. Those are all species I feel can be maintained in a smaller tank, provided there are hiding spots and water quality is good (both those requirements are basically covered by heavy planting).