We suggest that for your first 10 levels you go through the mudschool and the zones around it. There's enough there to level you to 10 and beyond.

After 10 levels:
Prehistoric:
Old Carthage (has some tough mobs) and Carthage Bay
Medieval:
Sanctuary Haze, Tournament Place
Old West:
Buffalo Bill's Circus
Future:
The Spaceship Titanic, Golf Course, The planet Endor, The planet Dagobah

And if the above zones become to easy but you still want somewhere not too hard:
Prehistoric:
Greek Archipelago, Ali Baba's Cave
Medieval:
Archers in Old Yorke, Ice Palace (will be a bit tough), NightDragon Realms
Old west:
The Range, Dirt Farm, Legend
Future:
The Gamegrid, The planet Tattooine, New Goznemia, Necromunda II, Planet Reptilia

This information is also posted on the Seekers board by Diandra.