Quite frankly almost anywhere would have plenty to offer but if you make it a day trip, I'd say that flight times, and link between airport and city centre would matter.

If it were me, despite the lack of originality, I might favour Edinburgh - frequent flights, fast transport to the city centre, which is quite compact and has a lot to offer. I'd probably avoid Manchester just because security is often a mess meaning one often needs to arrive a bit earlier than elsewhere.