We stayed near Plymouth 3 years ago and visited it for the day. We ate lunch in the Barbican area and took a boat trip. There is also an aquarium there, which we didn't visit because the weather was so fabulous. We also visited Dartmoor, Burgh Island, Lydford Gorge, Castle Drogo, Morwelham Quay, Tavistock, the Eden Project and Cotehele.
We go into plymouth when we stay in se cornwall as it is only about 25 mins away. there is a great cinema complex with bowling alley near to the aquarium if it is raining. there is a also a pub nearby called the china fleet where you can sit out on the balcony and look at the expensive boats. there are some fab national trust places like Antony, castle drogo and exeter is only about 30 mins away. there is devon crealy too as well as diggerland all less than an hour from plymouth. there is also a chinese eat all you want buffet place called the water dragon near the theatre but go in the evening early to get the chocolate fountain as it isnt on at lunch time. check out also the theatre royal to see what is on. how old are your kids?
We have just come back from a holiday in South Devon, staying in a lovely converted barn near Yelverton (just north of Plymouth)
We visited Castle Drogo, Morwellham Quay, The Dartmoor Pony Heritage Trust (dd sponsors a pony with them), had a day in Brixham, had a day in Dartmouth, walked on the moor (looked for but did not find Hound Tor which was apparently the inspiration for the Hound of the Baskervilles Sherlock Holmes story), had a picnic at Dartmeet near the stepping stones and many other fab things. We ate near the aquarium in Plymouth (Nando's, birthday treat for ds)