hey cargirl
what about alnwick, for alnwick gardens, brilliant for a whole day? is that too far north?
also, for great seaside outing i would second saltburn, or robin hood's bay which is so very gorgeous.
if you're near harrogate then brimham rocks as others have said, is fantastic - you could combine that with a trip to fountains abbey which is beautiful and a lovely walk around the lake.
newby hall, just outside ripon, is also brilliant, but would probably be good for near to a full day rather than part of one.
bolton abbey is another great place for a walk, scenery, splodge in the river. you can get the steam train from there to embsay and back, and also very close to there is hesketh park farm which gives you a full day out!
if you are used to chessington and legoland do not, on any account, go to lightwater valley!