Explore @bristol is good for most of a day if you haven't been before and try everything out - it's a couple of months since my last visit, but we can easily spend an hour or two in the cartoon-making section, then have a go at news reading in the tv studio, and jump about a bit in the virtual volley ball game. Oh, and the human hamster wheel which drives the water power thing is good, though there is often a queue. I haven't seen the new human body exhibition, but it sounds good.
If you want to combine it with anything else, there is a new aquarium next door (Blue Reef) - I haven't been, but DS went with school and said the octopus was cool. There's also the SS Great Britain not far away, if he is of a historical/engineering turn of mind.
Or if you are there at a weekend, the Pirate Walk leaves at 2pm from just outside @Bristol and is a fun thing to do.
Other tips - there is a cafe as part of the @Bristol building, but there are also loads of other restaurants within a few minutes' walk: Pizza Express around the corner, the Firehouse across the square (v good pizza, also grilled stuff), and several bar/restaurant places. Nice big Chinese restaurant too. Or there are plenty of benches around if you take your own lunch.