Bank Holiday Monday in London or Herts

[MrsAFlowerpot]

Hi all,
DH has an unexpected day off tomorrow and it looks like the weather is going to be lovely. We were going to take DD (4) to London Zoo, but it seems so expensive for what's on offer. Searching around it seems there are no events on really in London or Herts tomorrow that are (young) family friendly, which is a bit of a surprise on a bank holiday. Does anyone have any sunny suggestions? It is a rare day given we are both off and sun is due to shine!
TIA

I'm sure your DD would love London Zoo, but Whipsnade is bigger and you get more animals.

Alternatively, Battersea Park has a playground and a small zoo, and Hyde Park has the Diana, Princess of Wales Memorial playground, which is excellent and has lots of fun stuff inc. a huge pirate ship, but it will be busy on a hot bank holiday Monday. It has a nice cafe too. Perhaps you could cool off in the Diana fountain afterwards?