we live in Plymouth and this is great to get the kids out for the day. Although I must add that the free return doesn't count on first 5 bank holidays after getting it. for anyone able to go an extra couple of hours down the eden project does the same free return for 12 months.
We went to Plymouth last year mainly to visit the aquarium before going across to Devon the following day. We were so impressed with Plymouth we've booked a short 3 night break in July for my birthday, really looking forward as I loved the Barbican waterfront and rest of the coastline with the lido and all the little bars :) By all means go to Plymouth aquarium but it's Plymouth you'll remember ;)
if you book online prior to your visit, it's slightly cheaper and you still get the years entry included.
Good deal and Plymouth is a great city to visit.
Went there last year - annual pass has been in place for a long time now! Didn't think much of the place - THE DEEP is still the best. Even my kids agree on that!