Pokhara is the ultimate R&R destination in Nepal. It's the jumping-off point for many Annapurna area treks. Logistics usually require an overnight here, and adding a couple of days at the end of a trek is a great idea. Trekkers to other regions and visitors who aren't trekking should consider Pokhara too.
There is an old town and a day's worth of sightseeing to interesting caves, waterfalls and temples. But sightseeing isn't the point of Pokhara. It's a place to just chill out with the great mountains dominating the landscape: beautiful views and lots of nice, inexpensive restaurants and clubs.
Pokhara is also a hub for ultra-sports activities. There are Hang Gliding and Ultralight Flights and Zip Lining course (one of the longest in the world). There are two excellent options for dawn hikes to watch the sun rise on the mountains.