For my son's 3rd birthday party we had a FISHING PARTY.  The invitations were hand made shaped like fish with a bag of gummy worms attached to them.  At the party we had a fish pond where the kids got a small toy. We also had a large Jell-O tub filled with Jell-O and whipped cream. the kids had to find all the hidden gummy worms hidden throughout the Jell-O.  we had food related to fish and fishing. (tuna sandwiches, fish crackers etc) But the real hit of the party was (get parental permission first) a paddling pool filled with real gold fish.  The kids took turns fishing their prize out to take home. I purchased small fish bowls for each but you could give them baggies to put their fish in.  Everyone loved it!!!