You could put any ONE of the following options, but this is by no means all of your options:
1 betta -or-
2 female + 1 male guppy -or-
2 female + 1 male molly -or-
12 cherry shrimp (they will reproduce insanely, but reproduction can be controlled by amount of food given) -or-
1 african dwarf frog -or-
cardinia yellow shrimp -or-
neon green shrimp -or-
2 females 1 male endler's livebearer (they will reproduce, but again, food control and parents will eat offspring, so that controls it also.) -or- Many many other options.
You're doing great by asking rather than overstocking and then asking how to fix it.
Good job!