If you're in the area of this trail, the Northern Rail Trail is really great ! It is LONG (if you want a long trail - you don't have to ride all of it of course), and is nice and straight. The ride is pretty, and there are some interesting places to stop along the way.
The Cotton Valley trail (Wolfeboro at one end) is somewhat difficult for bicyclers because the trail itself actually crosses the tracks MANY times. Where these crossing exist, it is generally necessary to slow WAY down to cross safely or you risk a serious fall.
I like to ride a tandem bike if an able and willing female partner is available
and with this (long) bike it is absolutely necessary to pretty much stop dead and walk the bike over these crossing areas.
On a single bike, some people ride carefully across the rail crossings, but you have to approach it at almost a 90 degree angle to be safe, and sometimes there isn't enough room to do that. In any event, you have to slow way down and be careful.
I so wish someone would fix these crossings. I would donate time and/or money to the cause. The trail would be SO much better if these dangerous crossings were corrected.
Anyway, the Northern Rail Trail is definitely worth checking out if you're anyway near to it. I am at Mirror Lake (Tuftonboro) so it takes an hour to get from my place to the nearest point on the Northern Trail - but I do it sometimes because the trail is really good.