I know this isn't something new or fancy, but it is good old clean fun (except for the clean part). One of our favorite springtime activities is going out on a rainy day (light rain, mist, or just after rain seems best) and jumping in as many puddles as we can find.
Heading out for a walk in search of puddles.
Rain pants and waterproof boots highly recommended!
Another thing the boys like to do is to fill puddles with rocks. If your child doesn't like to get as wet, this is another great way to play with the puddles!