Its hard to beat the Little River Trail. Its a nice hike along some great water. You could stop at anytime and start fishing. If you were going to hike/camp you could stay at the Elkmont Campground (fee) or venture up to Backcountry Campsite #24 (reservations req'd) or Campsite #30.
Another good option is Deep Creek from either direction (Newfound Gap Rd [harder] or Bryson City[easier but a longer drive from Townsend]). Again there are a number of good campsites (Frontcountry and Backcountry) and great water to fish.
You could also hike into Abrams Falls from below on the Little Bottoms Manway. The fishing is not really that great until you get to the Falls and above, but the hike is much less crowded than the Cades Cove approach and there are a few campsites along the way as well.
All three hikes are back and forth hikes, so you could decide how far you want to hike and then turn around. So the length of the hike would be completely dependent on how you feel, rather than taking a loop hike and being somewhat committed to a certain distance to complete it.
How many days are you staying? Are you camping (if so, frontcountry [drive-in] or backcountry[hike-in])? I put together some info that you might want to look at that compares the relative difficulty
of some of the trails. Its not very scientific, but it might help you pick a few spots. If you can, pick up a copy of "Hiking Trails of the Smokies" from the Smokies Bookstore
. Its a great reference.