Well the South West covers a LARGE area! If for example you were near Ivybridge, Devon then I would suggest a trip to the beach (wrap up!) and do some rockpooling and then build a sandcastle! Bigbury on Sea is a good beach, there is an island which you can walk to at low tide or get the sea tractor at high tide. A walk in Longtimber woods, a trip to the swimming pool. Depends on the age of your children but mine always enjoyed a short train or bus ride. We used to get the train to a local town, have a burger, a look at the trains, a trip to Woolworths (for a 99p car!) and then the train home. That would use up a good portion of the day, keep them amused, and not cost too much. Then they always enjoyed baking, I made a huge batch of fairy cakes and gave them icing, hundred & thousands, glace cherries, chocolate buttons, desicatted coconut etc to decorate them with.