We took a day trip out to Swanley Park in Kent with the grandparents. If you don't mind travelling, it's well worth the journey!
We love it because... It's a park-come-theme park! It's HUGE so there's lots to explore, and little ones can choose from a mile-long train ride around the park (weekends only, we missed it unfortunately!) or a trip sailing on the lake. The best bit for us was the fantastic paddling pool and water fountains, handily located opposite the cafe for ice creams (they also sell swimming essentials like towels if you've forgotten to bring one!). The pool area is big so there's lots of space to play, even when the sun's out (unlike some of the more crowded inner-London water play areas, like Brockwell Park!)
We spotted... A train station! Weirdly realistic, it works as a stop for the park's train line. Our boys are big train fans so we'll definitely be doing this again! We also noted that they're hosting the Luna Cinema outdoor screenings this summer.
Top tip... There's a big car park here, so it's easy enough if you can drive. It's also really close to the motorway, so you could combine it with a trip to one of our Out of Town destinations.