You can always get a few of those big Bamboo Shrimp (Flower Shrimp)
They are large and passive.

There are also a number of freshwater crabs that are small and colorful. But you also have to make sure to have some surface above the water level, crabs need that. Also more specialized care is needed for crab but can be very rewarding.

I see "triops" for sale on Ebay all the time, but it reeks of scam.