There are relatively few days to use a swimming pool here in Alaska. We hit 72 and needed to cool off desperately. I found a Rubbermaid container to use as a kiddie pool for my six month-old, but needed something a bit deeper for my 21 month-old. I found a large cooler in the garage and it was wonderful. It kept the water warm (important in Alaska) and I was able to close the lid and keep bugs out when he ran around between dips.
I love this idea, but I can't post this without passing along a few obvious safety tips: supervise at ALL times (due to water play and accidental lid closure), and pack the ice chest way out of reach during non-playtime so toddlers can't climb in.