Feed them lots of veggies! It's great for the digestive system of your fish (: And they contain fiber and important vitamins!
I use to give my fish green peas (: They are very easy to prepare, just buy frozen, natural peas, thaw them and pop them out of their shells and you're ready to go!
Unless you have small goldfish, then you might need to cut them to some smaller pieces (:
Other things you could try would be big chunks of cucumber, zucchini, lettuce and orange (:
The lettuce could be "planted" in the grovel for your fish to graze on, or attached with a clip to the side of the aquarium. And remove any leftovers after 24 hours. Be careful with the orange too, take away all the peel so it won't get stuck in the mouth of your fish. Oranges contain lots of acids and juices, so be careful not to give too much of it, as it may pollute your water (:
Other than this, have fun with your fishies!