(Keep in mind, this might not be at exactly 18, but definatly right around there...I haven't played my druid for awhile, since the KEI fix)
There are a couple "off-the-beaten-path" places that you can solo in, besides the drama-ridden PC. (I do suggest to do at least one session in PC, at least to get the experience of this drama...then again, I may be a ******...)
North Karana will be a good safe bet. Good amount of mobs that you'll get exp on, wide open places to kite in, and best of all, a druid ring right there (you get Karana Gate at 19).
East Karana won't be too bad either, gnolls and spiders and stuff will still give exp. With spiders you get the added bonus of high drop rates of spider silks which you can use to skill up in tailoring, or sell in the bazaar for cash. Access will be easy once you get Karana Gate at 19.
South Karana will also work. Can't quite remember what's good at level 18, but I'm sure you'll find something. Also wide open and fairly flat.
Oasis of Marr will also be good up until about 20. Plenty of room to kite (my personal favorite was along the shore of the lake on the orc highway side....kite orcs back and forth along the shore, few adds but mobs close to pull, watch specs though). When orcs start to green out, go for the higher level caimans and crocodiles. (This used to be *THE* place to level pre-luclin, but now it's empty save for people going into LDoNs).
The Overthere should be decent if you go straight out from the PoK book (away from the fort) and hunt along the shore and west side of the zone. Large flat expanses make for good kiting.
Not quite sure where Luclin zones fit in here (Hollowshade/Shadeweaver's) but they may also work for you.
At a little over 20, it opens up more. Lake of Ill Omen and Stoneburnt Moutains (Kobolds near the Warrens) are both very good spots (stoneburnt has a lot of trees, might make it tough to kite). North Karana will still work fairly well at this level, as will South Karana. (I swear the Karana's were just MADE for kiting =p).
If you are looking for the fastest exp, the PC is your best bet, though the fastest exp isn't always the best ;)
*The kiting that I used was mainly snaring and dotting, then running around in circles. I had played a bard a bit when I started the druid...so I liked circles ;).