Calvin once attempted to jump off of a ladder backwards into his kiddy pool. It is unknown whether or not he actually did, as right when Calvin was about to jump, Hobbes told him he'd "Better hurry. (His) mom's yelling something".
Once Calvin and Hobbes put on their scuba gear hoping to have fun exploring coral reefs, but instead they could barely fit inside their kiddy pool.
Hobbes once did laps in the kiddy pool. However, Hobbes being longer than the pool is wide, this was no challenge.
Once Calvin was seen digging a hole a bit smaller and deeper than his kiddy pool, but gave up, saying "too bad, Mom would be really surprised to have an olympic pool where her garden used to be", to which Hobbes replies "maybe she'll still be surprised". Also, Hobbes once ran through the sprinkler, after Calvin wouldn't let him in the pool.