How old is your toddler?
We went to Elvedon in November last year with DD (then 2.9) and her cousin (then 1.11) and prebooked a lot of stuff which on reflection we probably wouldn't do again!
Stuff we booked:
Teddy Bear's picnic - this was fantastic, they both really enjoyed it (altho DC was terrified of Rupert Bear at the end and wouldn't go for her photo opportunity, much to my BIL's frustration) and there were lots of songs and games.
The panto-type show - something about Jack & Jill in the forgotten nursery - it was a lot better than we feared, but although DD loved the singing, she was otherwise more interested in the other children there and DC just wandered about, so a waste of money really! And it was PACKED!
We did the morning soft-play type session which we thought would be a tumbletots type thing (that was how they described it) but it was just a load of toys laid out for them with no real direction except for some nursery rhymes and songs at the end - they enjoyed it but again, not sure it was worth it.
Didn't get up in time to go to Baby Dolphins!
Messy play session was fantastic, would recommend booking that.
Tbh, the only thing I'd really prebook next time around would be the messy play and the bike trailer - NOT a child seat, it might rain and also it's really useful to stash your swimming things in when en-route to the pool.
Free stuff they loved - playgrounds, soft play in the restaurants, chasing ducks, puddle splashing and the pool.
We didn't prebook the time out club but were able to put them in one morning to give us the chance to have a proper swim...
Have fun!