One of my favourite subjects this, and I think I might have visited them all!
Scarborough is my absolute favourite, though be aware that your trip will be weather dependant because it can get pretty cold and windy there (though the decision to move the Festival back to a more sensible time of year should make this a bit easier).
Worcester you've done.
Chelmsford wouldn't be high on many people's lists...but the atmosphere is one of the best around, especially for a day night game.
If you get the chance, outgrounds can be excellent - Tunbridge Wells, Bath and Abergaveny are amongst my favourite (though I am not sure that Glamorgan still play there)
Oh, and don't forget Lords...