Forms plus money.
Mostly plane tickets cost a lot more then the price of a passport, and hotel rooms even more. However you can find the occasional cheep deal on plane tickets, and crash at a friends house. If you are doing a vacation on a budget I can totally see skipping the passport thing because it give an extra day or two of hotel fee, so you can stay out longer.
I mean I know a lot of people that pretty much only go on vacations to visit relatives…
