Loads and loads of things. DH and I were just saying the other day, why is it not a more famous holiday destination.
You can hire kayaks or paddleboards and navigate the Wey river.
Thorpe Park or Chessington World of Adventures
Go Ape at Alice Holt or Chessington
Bird world if they like birds
If they like hiking, climb Box Hill or Leith Hill (Leith has a tower on top you can climb too) or walk around the stunning Devil's punchbowl. If they are superfit, hire or take bikes and do the bike challenge of cycling up Box Hill which is Olympic training territory for cyclists.
If they like wild swimming, go out to Frensham Ponds, a fresh water lake with natural sandy beach, loos, showers, cafe and striking weird heathland all around it. There are outdoor lidos at Guildford and Hampton Court, a skating rink and climbing wall at Guildford Spectrum.
Surrey is stunningly beautiful. Loads of woodland for cycling and hiking trails or find a local riding school and book a trek.
Check theatres in Woking, Kingston, Epsom and Guildford for plays, comedians and gigs.
Hampton Court maze and palace are fun if they are interested in history. You can get a boat from Kingston. Loads of other stately homes including Painshill in Cobham which has beautiful grounds with ruined follies and glittering grottos. If they like plants (weirdly my teens did) RHS Wisley is a really beautiful place for a picnic. It has a massive tropical greenhouse with waterfall and jungle and desert plants, loads of lakes with some baby moorhens right now.
As PP has said the sculpture parks are good. There are at least two. You can spend hours in there.