Where do you fancy going?
I could see what we can book for Silverdale in May. Lots of camping, cycling, walking and other touristy things such as Leighton Moss RSPB, gateway to The Lakes, etc.
Gibraltar Farm has a group field which could accommodate 30 tents quite easily but also open camping fields. The facilities block is very good and there are even launderette facilities for the dirty beggars. There is also a touring caravan / campervan area for those who love their tin boxes.
Depending upon numbers there is a nice Indian restaurant, a couple of pubs, a chippy (I believe though we've never used it) and a Coop.