Another vote for Woodlands! Go early to make the most of it, and book online, it's cheaper in advance.
There are of course a million lovely beaches. Bigbury is great when the tide's out, and you can walk along the tidal causeway to the island. They also hire out boards and boats, and the cafe do good chips!
You're about 30 mins drive from Plymouth, if it rains, where there's a good aquarium, ice skating, and a decent cinema/bowlplex type thing. The Barbican is good, and if you happen to be down on the 12th/13th August, the British firework champs happen, 3 massive displays each night, and it's free! It's a late one - first display 9.30, last display finishes around 10.15-10.30, but totally worth it.
If you want to be a bit more cultured, Mount Edgecome is lovely - you can park at Devil's Point and get the passenger ferry over (around £2 each I think!). Don't bother with the house, the grounds are extensive and gorgeous and free.
There's also Dartmoor Zoo, and probably a lot of other places, but these are the ones that instantly come to my head!!