You could easily ride all day at
1. Mount Snow, VT
If you did two climbs from the base to the peak in one day and took time to explore other trails on the way down I would consider that a solid day. I usually do one climb in the AM and one after lunch. You can throw in some additional climbing if you still feel saucy. You could even ride the lifts if you had to
2. Killington, VT
same as above.
3. Kindom Trails, East Burke, VT
100 miles of trails advertised. Some of it not worth riding but most of the singletrack is sweet. I would suggest trying to ride all the singletrack available in one day.
These three spots are very well known and certainly no secret, but they still provide great and adequate terrain to ride a full day.
I am sure there are many more in NH and that the Locals keep secret also.