Ruislip Lido is a great place to visit during the summer, with lots of (FREE!) outdoor fun to be had; including a splash park, sandy beach, large play area on the sand, woodlands and a train ride (charge applies for this).
If you are visiting with young children, or those with SEND, please keep in mind that the play area is next to open water and not enclosed. Also, no paddling or swimming is allowed in the lido itself.
We think Ruislip Lido is a lovely place anytime of year (although please note it is practically entirely outside, but we don’t mind- that’s what waterproofs and layers are for!) For further information, visit their website here.
Of course there are many others in and around Hertfordshire, this is not a comprehensive list! Others I have heard of but never visited are Harpenden outdoor pool, Letchworth splash park/ paddling pool and Hitchin outdoor pool. Plus, of course, there is natural water play such as the stream at Wheathampstead, Stanborough or Cassiobury and plenty of beaches within an hour or two (driving or train) too!