I go with ITJock on this one.
A set of adjustable dumbbells (not the fancy selectorized deals - just interchangeable plates) - two or three sets if you can afford them - is a great start.
I have a number of clients who travel extensively, so I've designed dumbbell-only workouts for them, since dumbbells are the one thing you can expect to find in any gym, anywhere.
If you have the room for it, a decent quality adjustable bench wouldn't hurt, either.
Buy them used, if you can - check the local Pennysaver or Craig's List. Residual value of exercise equipment is pretty low.