One of my favorite loops from LGNC is to climb the AT away from the center, in a southwestern direction, then hike out the until you get to the ridge, which seems far (stay on AT, don't turn off it). A little bit out the ridge, maybe to the farthest TV Tower, then turn around and come back east. Instead of coming down the AT, however, stay up on top of the ridge and walk back through the 'high meadow-y' part of the mountain, a neat environment with some good views to the North, that you don't get from the AT. Sput trail leads out to an overlook, if you want. Eventually, the trail comes around and rejoins the AT on your way back down to the nature center. A real nice area, a good workout, not a lot of traffic, yet close to civilization.
Lehigh Gap Nature Center offers some cool hikes from their location at the base of the Blue Mtn where the Lehigh River cuts through. The AT comes right down to their building, and with the other trails on their grounds, a couple neat loops can be made. There are maps available.
http://www.tripleblaze.com/trail.php?c=3&i=1373