How to entertain a baby in the heat

I'm from the UK and we're having a mega hot summer. Probably nothing compared to what some of you have on a daily basis. I've got a 4 month old son who gets bored of his toys and playing. It's too hot to take him out for a walk which then gets me bored too! As it's summer the baby groups are limited too. I'm running out of ideas to keep him entertained and cool.