Sounds like you're off to a great start. Unfortunately there's not much in the way of tankmates that can go with a betta in a 3.5 gallon tank. I have tried putting shrimp in with many of my bettas over the years, and inevitably they have always gotten eaten, even though all of the tanks were heavily planted. Bettas are smart, they will seek and destroy your shrimp, lol.
You could get some ghost shrimp, they are only about 10 cents a piece, that way it isn't a big monetary loss if your betta eats the shrimp--however most people aren't really comfortable with the notion that they're buying animals that will eventually end up being food for another animal.
You could always get a snail--zebra nerite snails are pretty and they can't reproduce very well in fresh water. They'll help with the algae too.