In my late 30's I did a lot of questing in the Wild Lands of Zelata which worked out well - theres a lot available there. I did Sanctum of the Burning Souls twice, but the single mob pull nature of the instance didn't make for much excitement. I did get a few nice rare drops though which should last me a while.
Around 38-40, I went around to all the areas where I had "Deliver this item" type quests, mostly down in the Stygian region which I had yet to go to. It was a good thing I did because I had an incomplete part of the Destiny Quest line from Knopshef Province...oops! I did a few quests in the area since I could fly through them. Even though a few were green or even gray, they still provided decent XP. I then worked my way over to the Pyramid of the Ancients and the Treasury of the Ancients. Unfortunately, the Pyramid is bugged out. The 'Enter the Pyramid' quest never gives you credit, which I'm sure has a followup to it to explore further in, and the quest later on to collect the eyes for the statue never gives you credit when you turn in the eyes, it just offers you the same quest again. Oh well. At least the Treasury was a fun event. Kind of a stamina type event with statues coming to life every 20 seconds or so.
By this point I made my way to the Fields of the Dead. On the way, I soloed a few gray elite quests in my quest log in Conall's Valley which had been lingering far too long. Groups never seemed to make it that far back in the elite region to ever complete them. Slow going, but good XP actually.
Finally, I entered the Fields of the dead at level 41 I believe. I worked through the tons of quests available, which quickly turned all orange, red, and orange elite. I struggled through the orange ones fighting mobs 2 and 3 levels higher than me. One mobs at this level I could handle, it was when I got 2, especially one melee and one ranged where I had to move them around to be able to do AoE, or knockbacks.
I am now 45, and only have red and elite quests left in the Field of the Dead. Not really sure where I will head to next - the region is listed as 40-50 so maybe I'm missing some quest givers. I'll have to check it out. I also have a couple quests in the resource region (name escapes me now, Zaara something) that are green elite. Might try those out before struggling again in the Fields of the Dead. I also picked up the alchemy quests which I can do for a little XP.
This is the first time where I don't have an abundance of quests to do where I have to plan out a route to maximize quests completed. Hopefully this doesn;t last too long.