In a word (two words actually): folding tables. The pre-built kind, which you can get through Amazon and lots of other places. Adhere your pink/blue foam or other subroadbed/scenery base to the tabletop, and you've got an inexpensive and (probably, choose wisely) lightweight layout that folds up when not in use.
I've noticed there are some common sizes which seem particularly well suited for N scale, including 30" X 72", 2' X 4', and 34" square. There are even some round folding tables, but they're pricier.