Sabai Sabai is a good Thai restaurant.
Restaurant Het Prentenkabinet
Restaurant 't Crabbetje (seafood)
There is an Italian near the haven that I can't remember the name which is very good.
Great bars are:
The Bonte Koe (character)
The Burcht (Jazz & Character)
Maneer Jansens (character)
Lemmy's (great selection of beer)
Cafe Barrera (nice setting)
De WW (studentish, good music)
You could try out the Leiden expat group also, they sometimes organise dinners out in Leiden.
http://groups.yahoo.com/group/Leidenexpats/