Where are you staying and do you have a car?
The zoo is great and you can spend all day there. Blue Planet Aquarium is pretty poor and you'd be hard pressed to fill an afternoon.
There's a couple of escape rooms, although you can visit one of those anywhere, of course. I've been to Escapism a couple of times with kids and they love it.
Grosvenor Park and the river are lovely for a walk round and to buy an ice cream. You can walk round the city walls in about 30-40 minutes, if you fancy that. Good range of shops in town, and 'The Rows' (one row of shops on top of another) are unique. Loads of nice cafes in the city centre. The cathedral is lovely, with nice gardens and the beautiful Abbey Square is worth a look.
Storyhouse is good for a coffee or lunch, and there might be a show on, or a film showing. The library occupies the same space as the cafe in storyhouse and it's got a great calming vibe, for when you need to recharge.
If you're there in the Summer, the Open Air Theatre (run by Storyhouse) is putting on Jungle Book, Merry Wives of Windsor and Pride and Prejudice this year. Their productions are usually really good, you can take a picnic, and loads of families go to these plays. They also run an outdoor cinema, called Moonlight Flicks. Not certain if that is on this year.
I'd definitely recommend a walking tour of some kind - lots of options suitable for children, including a ghost tour that runs in the evening.
Have fun 