We have stayed in a cottage in Cornwall which was so atmospheric that it really didn't matter that it rained because it gave us the chance to not feel we had to go out the for the day!
We have only ever paid for one attraction in all our years of going and that was the Eden Project and that is terribly expensive unless you have vouchers.
We have spent days on the beach at Rock (no cost and parked in a field opened by a farmer) eating pasties (avoiding the prices at nearby Padstow). Padstow is terribly packed.
There is also a vineyard on the way there with wine tours and a bottle was left at our holiday cottage.
Kennack Sands is lovely and sheltered with cafes on the beach. Again fields are open for parking free. You can also walk to Kennack Sands from Cadgwith which is also lovely and the walks along the cliff top are spectacular. A submarine was sailing towards us last time we walked there. There are wild ponies on the cliff top.
We also went to Church Cove which we found somewhat spooky on a misty, windy day along with the Church.
There is a weird Victorian town attraction nearby I think it may be called Flambards.
There are lovely villages along the Helford River and nice pubs and ferry crossings.
Moving into Plymouth, there are boat trips which are cheap and cheerful then lunch at the greasy spoon on the marina front. I think it's called Captain something or other.
Visit the South Hams, if you don't mind travelling and go to Salcombe. Again, nice beach and you don't have to spend anything but outrageous property prices.