I had the pleasure of teaching down there for a term. Although a cold and wet time of year it was just the most amazing place.
I lived in Plymouth for four years and loved it. Went back over the summer and stayed just round the coast.
Family holiday wise it is great - the national marine aquarium is superb and I always liked the Barbican. Also, for a family day out, Pennywell farm was superb with my 2 year old.
However, as a place to stay, extend your area to over the 'border' (the Tamar) and go somewhere like Looe. Or North Devon where the beaches are even better. Just avoid Padstein (Padstow).
Have to disagree with you about Padstow,it is my favourite town in Cornwall and is far more than just Padstein.It is a beautiful area with plenty for everyone.
We do avoid visiting in the height of the season though, as like many other places it gets packed.
My wife and I are staying in Brixham for a few days in mid September and then off to Padstow in the second half of October!
Many good suggestions have been made for the O.P. and I hope he has a good holiday.At least he is unlikely to bump into the rather blinkered 'Complete Badger'!