Newlands Corner isn't a bad bet except it'll likely be quite busy (as always). There are a few little routes around there that can be good fun but it's working out whether a convoy of 25 cars is really appropriate for the little (and I mean little) roads. There's no decent dual carriageway stretch short of popping down to Clandon Lights?
Well if we're taking down the A25 towards Dorking, we could always turn left opposite the turning for Shere and head over the hill towards Clandon.
I think it does depend on turnout tbh, there are a lot of cars to accomodate.