We did techniquest recently with my kids, not sure a three year old will get much from it but the seven year old will I'm sure.
Quite a few of the exhibits have been revamped from when I can remember going as a kid.
We also booked a live talk in their theatre, one about going to space, that's was good too, they lit various gasses and made a rocket 'fly' across the room. I think that was an extra £3 each.
As for town / hotels I can't help.