Even if you don't learn to actually speak a foreign language, learning to read and understand it can be immensely helpful. And even if you're not going to travel for a few years you can start getting a feel for these languages now. Are there any Spanish-speaking radio stations in your part of the USA? Tune in to them from time to time, you'll be amazed as to how much you pick up just from announcements in between records. You can probably find other foreign-language stations either on Short Wave or on the Internet.

Of the places/language you mention I'd go for Spanish first, then French or Italian, Dutch last. The Dutch expect to use English with foreign visitors, and I have only ever once come across an adult Dutch person who didn't speak English - a bus driver many years ago. In Spanish-speaking countries, on the other hand, you will come across people who don't speak English, and you will need to know some Spanish for everyday transactions such as buying things in local shops.