Mt DDs both went to Berkhampstead on Pittville Circus Road - very friendly, happy school, almost 50/50 boys and girls. Good academic record too - they get several scholarships most years to local independents, inc CLC and the Boys' College. Grammar places too. Overall a really lovely, happy school but perhaps a tad outdated in facilities (not that that's everything by a long way and they have done a lot of work since mine left, but essentially the site is a little small.)
More 'modern' (at least in buildings) is Richard Pate's (no connection these days to Pate's Grammar) and has a fantastic reputation - also co-ed - good grounds etc.
Dean Close prep and pre-prep - beautiful surroundings, unparalleled facilities. DD1 went on from Berky to the Senior School there and while it was an excellent school she wasn't particularly happy there (no reflection on the school really) - the pre-prep and prep are lovely.
The Boys' College also takes girls now in the pre-prep and prep, I understand. Lovely grounds and good reputation.
There's also St Edwards - another co-ed school from 3-18 with beautiful facilities and surroundings - co-ed - used to be thought of as 'less academic' than CLC or the others but that may not be true nowadays.
You're spoilt for choice, really! There are others, too, but I think those are the main ones.