Birthday outing

We’ve been talking about where we can go for the day on Dylan’s birthday. What we’ll enjoy and more importantly something he’ll enjoy.
We crossed off London zoo because we’re unsure how the weather will be. We’re considering London aquarium because of the sensory aspect and as it’s on South Bank if the weathers nice we can walk outside.
I quite like the idea of going to Hamleys to buy a ‘special’ birthday present.
The problem is entry to the aquarium will be almost £40 for Harrison and I (Dylan has free entry) and although money isn’t really an issue I’m wondering how much Dylan will appreciate a proper day trip when I could spend that entry fee on another nice present.

I’m at a loss for more ideas for things we can do.. Any ideas would be amazing.