Rain is not a disaster in Poole, which has more indoor options than its beach reputation suggests.
Free and indoors
Poole Museum on the Quay has historically been free, spreads over several floors, and holds the Iron Age Poole Logboat.
Something booked
Lighthouse, Poole's Centre for the Arts, runs a concert hall, theatre, studio and cinema screen under one roof. See theatres and check what's on.
Eat it out
A long lunch is a legitimate plan. Afternoon tea, the Old Town pubs or a table on the Quay watching the rain move across the harbour. See restaurants.
Shop
The Dolphin Centre is covered, and the Old Town galleries and independents are a better browse than the weather deserves.
Or go anyway
Wet-weather gear and an empty beach is its own argument, and the harbour under a grey sky is often better than under a blue one.