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Dinosaur party: If you already have a sandbox than just hide the "fossils" in the sandbox. If you don't already have one you can buy a plastic kiddie pool and fill it with sand and do the same. Mold your own bones out of clay that dries overnight and buy some paint brushes.
Here's two simple ideas I like - plastic dinosaurs for decorating the cake or the table; individual drink bottles or drink poppers, decorated with a printed label (look on etsy.com for themed party printables).