I usually do something like:

Rosedale → Federal City → Cold Soil → Blackwell Mills → Pennington Titusville → Bear Tavern → Wash Crossing Pennington → River Road to New Hope / Lambertville

Brunswick → you can stay on that until Hopewell → Princeton Rd → Cherry Valley → Great Road

I wouldn't worry about the traffic, except on truly insane roads like 31 (which I avoid completely). Maybe be careful before the Washington Crossing bridge, it's a little hairy there. I also prefer to use lights on 29.

You should also go up to Sourland some time, it's nice up there.