Underwater theme for a 3 year old:Lessons learnt: What impresses an adult is not what impresses a 3 year old and one should know the difference. 3 year olds are very literal (cute little hand made paper treasure chests don't impress them)and can differ greatly in their physical development (so no competitive games). They enjoy really simple, physical games (which may not make sense to adults). Most importantly- a party doesn't have to be perfect to be fun. That said, here is what we did for my daughter's party:(especially check out my sections on games and the cake). Decorations: Streamers & Cardboard fish: from a scanned watercolour picture of a fish and colour printed in thin pastel coloured board (which means we didn't have to colour anything by hand). Keep the shape simple so they are easier to cut out - because it is quite a job cutting out 100's! Manta Rays: made from leftover polystyrene wine bottle separators, to which we added simple cardboard tails, paper eyes and ribbon to hang. Tables: blue cellophane sheets as table cloths. (All these completely lost on the kids of course!)Paper plates to which we stuck a round scanned and then copied picture of fish. Plastic disposable cups - to which we also stuck fishie pictures. But we used the wrong glue and it ate right through the cups! The cups ended up being used for the games as markers. Food: For the kids:(note, you don't get blue jello here) Wiener octopus (idea from this site), shrimp flavoured crisps, tiny plastic bottles of sherbet, a shoe box covered to look like a treasure chest with elasticised sweetie bracelets and chocolate coins (contents of the chest was a hit,box was lost on the kids.) Marshmallows shaped as ducks(passed as seabirds). Small sea creature "jelly" sweets and lollipops. Blue candied popcorn.To drink: a commercial brand of blue cooldrink used by sportsmen. For adults: 2 types of fish pie, dough fritters stuffed with mock-crabsticks in sweet chilliesauce,salmon and pineapple sandwiches, chocolate pralines shaped as shells and presented in chocolate shells, meringues shaped as shells. Blue punch served in a fishbowl with ice-cubes with jelly-sweet creatures in them. Somehow the alcohol in the punch ended up disintegrating the jelly sweets though. The cake:a 3-D fish surrounded by blue candied popcorn. Seaweed made from coloured liqourice strings kept upright with coloured pipecleaners and stuck into an upside down paper plate. This was pasted with icing sugar onto the cake's serving board and covered with popcorn.The cake was made from 4 separate smallish round cakes (from commercial cake mix). For the body 2 cake rounds were stuck on top each other and pieces cut off on two sides to make the body less round and more streamlined. The side fins were each made by taking a quarter of a cake (ie the 3rd cake round was first cut into quarters) and sticking it upright and at an angle to the main body (i.e. cut side down almost like a boat's sail). For the tail I cut the 4th cake round in half and pasted the 2 halves, cut side down, standing upright at the tail end. It was then iced (which was a laugh!)and decorated. Everyone loved the two round "jelly"-sweet eyes that were stuck down with toothpicks. Note: before you cut the cakes, put them in the fridge overnight so they don't crumble so badly. I was relieved that the cake worked out. Games:Learn from my experience...No fancy treasure hunt with clues - a simple easter egg-style hunt on a fishy theme is fine. We rolled big gym balls round in the garden (pretending they were pearls). Spun around until we got dizzy (pretending we are trapped in the sea currents). Climbed inside open-ended cardboard boxes (pretending to be in underwater caves hiding from the sharks)(I suppose you could decorate the boxes but we didn't since this was a spur of the moment idea). Wheeled smaller plastic balls around obstacles in a toy wheelbarrow (as if the fish had to move all her eggs).Also, you could drop lots of plastic balls onto the ground and get everyone to throw them back into a big box/plastic pond as fast as they can ("the sharks are coming- quickly, hide the fish eggs everybody')  Favours: The treasure hunt ended with a gift bag (made from plastic net bags used for packaging vegetables)filled with a fish-theme wind-up toy, sea stickers and sweets)with a photocopied thank you note stapled to it. You could also make little fishing nets from the same netting and wire coathangers - but I didn't get around to that.  All in all, pretty hard work but great fun.