It appears impossible to rest at all in the 3rd level catacombs under Candlekeep (the one with Prat and the Greater Basilisks). Every time you're interrupted by Phase Spiders.
Yeah, Ankhegs are nice. 1k XP per kill and easy to defeat even at low level.
I prefer WInter wolfs (987xp) in Nashkel mines....the pelts don't weight a ton like the Ankheg shells and go for 500 as well. Only downside that also goblins might spawn, but nothing a handy quickload can't manage And it's nice to *erm* grind/farm with lvl1 and buy all your crappy stuff with lvl2/3 ...
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For XP only the other choices are more common.
But perhaps there are better possibilities in BGEE.
edit- actually i've no idea how low level you can arrive here, would be interesting to give it a go.
Also @merlance and @greaterlion where in durlag's tower can you sleep to spawn greater doppelgangers? Very interested in that.