Welcome to Kernow!
I'm not far from Bude so can recommend the following for indoor/fun away from the beach...
Bude has a indoor pool and bowling alley/massive soft play (for up to 11years iI I think) with pool tables and a small arcade. Opposite Morrisons.
Bude Castle has a museum/visitors centre which is free and really interesting and well done.
Sandymouth and Hedley Bridge holiday parks allow non-residents for swimming for a small fee - call them to check
Rebel Cinema is a lovely independant cinema just south of Bude
Penhallam Manor - woodland walk with rope swings leads to the ruins of a medieval manor. Nice and sheltered for if its windy.
Discovery42 in Bodmin and Bodmin Jail
Lanhydrock is a really good, interesting NT property - kitchens, servants quarters and the nursery/school room are evocative and more relatable IMO especially for children
Cardinham Woods - play areas, bike and walking trails. Great cafe.
Wadebridge - nice for a mooch, cool vintage shop, massive park, cute old fashioned cinema.
Strawberry Fields Farmshop at Lifton - animals to look at, incredible farm shop for stocking up on goodies, strawberry picking, play areas. This is near Launceston just off the A30
Hope that helps!