I second the Ellicotville idea, we did it with a group last summer and was great. there is easily two full days of riding there and probably more like three with a bigger group. Its a ski town so there is lots of houses to rent, thats what we did and were able to ride right to the trails from there. They have a great map of all the single track as well, was just updated late last summer. Excellent mix of singletrack, not too technical in most places but a blast to ride.