From 16 - 20 so far, I spent a lot of time in Upper Guk. I am thinking I can stay here until 25 and still have meaningful experience gains (although I am sure one could stay longer). As I try out additional camps, I will add my experiences. There are numerous camps in the zone which are great for these levels.. There's the Scryer camp (Drops Bracelet of Woven Grass (common), Shimmering Orb (rare) and a Glowing mask (Enchanter quest item). I also was in a camp at the warden area. He drops Elf Hide Gloves as a common and the Troll Hide Belt as a rare. The only other camp area which I have been to in the zone is at the zone line to the undead side of Lower Guk. There are plenty of higher level Frogloks to pull to this area and there's the benefit of the zone line right behind you if needed. I previously also hunted the Ancient Croc but I can't recall at what level (it was back in 99 / 2000)
Here are a couple of good maps for the area..
http://www.msu.edu/~oconne53/eqatlas/uppergukmap.html (best detail)
http://www.steveprutz.com/eq/gukupper.jpg (less detail in terms of camp locations but a little easier to follow for navigation.