Crealy is so good at that age - I don't like theme parks usually, but as it doesn't have rides for older age groups, it's always been queue-free and has appropriate rides centred around younger children.
There's also Diggerland, if DS likes diggers. Escot is nice for a day, the Saxon village thing there is fun, good play areas. Nice cafe. And of course, the animals.
At that age, DS loved Devon Railway Centre, which is about 20 mins drive north, in a lovely spot near the Exe for a bit of a wander in Bickleigh afterwards.
Exmouth and Sidmouth were probably the beaches we used most when we lived there. The train to Exmouth is actually really nice, as you get good coastal views.
If you like stately homes, Killerton is actually really nice for small people: the bear hut, lovely grounds and gardens.
On rainy days, the museum in town is really good.