You have the National Marine Aquarium at the Barbican which is worth a visit, especially on a rainy day. The whole Barbican/Plymouth Hoe area is lovely on a nice day, with pleny of places to eat & drink, an outdoor swimming pool and a couple of kids play areas at West Hoe. There's a ferry that runs from the Barbican to Mount Edgecumbe in Cornwall, and from Cawsand (a pretty little village on the Rame Peninsula with a nice little beach) to Royal William Yard (an old RN victualling yard now stocked with bars/restaurants). There's shopping in the city centre if that's your thing, although I personally think Exeter is a better place for that sort of thing. And while the Hoe and Barbican are pretty, Plymouth city centre is a bit ugly to be frank.
Outside of Plymouth, in both Devon & Cornwall, there's a number of good beaches, both quiet places with just sea/sand/ice cream shack and busier places like Paignton. And of course Dartmoor and Bodmin Moor, which are beautiful places with a wide choice of good country pubs to stop at.
If you're down for a week and don't mind travelling a bit further afield there's any number of pretty little towns and villages to visit, from Dartmouth & Salcombe in South Devon to Looe, Polperro & Fowey in South East Cornwall or Newquay and Padstow to the North.
There's a couple of theme parks (Woodlands near Dartmouth and Crealy near Exeter spring to mind), or Paignton zoo & Living Coasts in Torquay.
Depending on where your cottage is, pretty much everywhere I've listed above will be within 1 - 1 1/2 hours by car at most.*
*depending on holiday traffic. Grockles drive slowly.
But agree, I've been on holiday here for over 30 years.
If into fishing day trips out into the sound, done that once, good day out, but no need to repeat.
also grockle boats trips around the harbour.
Famous Plymouth Gin distillery on the Barbican.
Get onto the internet and Google the area, lots to do that may even entertain your children's age span.
West Hoe has a park with crazy golf if that's your thing.
Deck chair attendant, tuff job but someone has to do it, Christmas tree seller later in the year.
When I tried to put tuff job it auto corrected to tiff job, may be my spell checker is not thick afteral?