What are the best cool places?

[SquirmOfEels]

I mean literally!

Lidos and outdoors swimming areas tend to be rammed from very early in when the weather is hot. Are there any hidden treasures a bit further out (say, still within M25)?

Or just good things to do when it's ridiculously hot outside, and you really need something different to do/see. Not just outdoors things, anything that's not likely to be stuffed full of other hot and cross visitors.

)And how far up the Thames do you need to go to find boat hire that won't have sold out by 0900?)