Everybody has their own tastes of what is good and what is not.
I personally really like 20-30 in Duskwood and Wetlands.

If your boxing, instances are always a good option.
They're practice for later on.
And generally give very good gear for your level.

If your boosting, 1-60, will be so fast you won't be bored anywhere.

If you're not boosting give yourself some variety.
Instance a bit.
PvP a bit, even if its not leveling until 3.2.
Quest in a few area's.
Possibly grind on something that drops cloth/leather.
But if you grind, have 90% of your attention on the TV or DVD.

Anytime you need a break, take one.
The game will still be there in an hour or a week.
Don't get burnt out if you can help it.