Hungarian.

Neither Germanic nor Latin/Romantic, so it'll be a truly new language for you. Plus has the best food.

After Spanish, Chinese will probably be most useful to you in the long run. Especially if you live on the West Coast of the US.

Russian, a beautiful tongue. Also probably pretty useful as well.

If you're going specifically for usefulness, go with Chinese as Tolstoy suggests. If you want something easy to learn, German and French are both good options, as are anything from those families. If you don't care at all about usefulness and just want something cool, learn Latin or Russian.

I'm going to recommend Mandarin (Chinese), Japanese, Latin, or Russian as good choices. I've never really liked how German rolled off the tongue, personally. Given your current fluency with Spanish, Latin would likely be easy to transition to, and it helps a lot in intuiting the meanings of other Latin-language-family words found in several languages. Chinese is probably the most immediately useful language you could learn and opens an entire culture to your study as well. Japanese can be immensely rewarding if you enjoy any other Japanese media, and (from what I've heard) is markedly easier to learn than Chinese due to the use of kana as well as the symbol Kanji/hanzi. Whereas Russian is more Western-leaning, another significant world power's native tongue, and useful for all sorts of study.

Of course, if you're feeling really adventurous, study Arabic.

If you want to learn a language for personal interest, then go for it. Russian is certainly a nice language and one I wish I had looked into. I studied German in high school and college, spoke it pretty fluently for a while, although it took me many hours of practice and a 3 year stay abroad to get there.

If you are trying to learn a language for competitive edge in the job market, then your time and effort are probably best put into other things.

http://freakonomics.com/2014/03/06/is-learning-a-foreign-language-really-worth-it-a-new-freakonomics-radio-podcast/

I certainly agree MM on the last. If at all, you should just decide in which field you'd like to work and in which countries you'd like to go. In engineering it is probably a good choice to go with German or Japanese. German opens you the opportunity of three highly engineering-leant economies and Japan as well. I would say the same for any chemical profession (but Valis is probably better informed). The truth is that in German-speaking companies you will easily get opportunities even if you can't speak German at arrival. They do expect you to be fairly fluent after a year the least. In Japan you won't have chance unless you speak Japanese but here again, you won't make career in Japan in a Japanese company in all likelihood.

In any other case, you should just go with what interests you. As an American you will not need any foreign language to have economic success in life.

Fortran

I have to say that Spanish or Portuguese are quite useless languages to learn over here in Australia. In Nth America sure, I see the value of learning these languages but over here? nope. Languages that are valued tend to be Asain (ANY Asian) language. The value of a language seems to lie in the amount of foreign language speakers firstly in your immediate environment, and then secondly if you have aspirations towards working/living in a foreign country, but even then learning a language is easiest to do if you are actually living in an environment that immerses you in it.

Esperanto

The six official languages of the UN are English, Spanish, French, Arabic, Mandarin, and Russian. Hindi (and a lot of other languages) have more native and secondary speakers than Russian, but Chinese and Hindi are relatively geographically concentrated.

It depends on why you want to learn it. While I am on record as rejecting ethnicity as a meaningful concept, if your grandparents are Italian or Japanese or whatever, you might feel like learning that for the family connection. If there's a

For practicality, I would learn French or Arabic. If you can speak English, Spanish, and French or Arabic, you can go almost anywhere in the world, especially outside of southern and eastern Asia, and find someone you can talk to. For enjoyment and learning for the sake of learning, I would go with Greek or a dead(ish) language like Latin.

German & Chinese.

If you want to avoid spending time in China and being a road warrior for your career, then avoid learning the language. Otherwise, you'll most likely be living in China or on an airplane away from family.

Depends on what your major will be. If you are going into political science: Spanish, German, Chinese, Russian, and Arabic are big. If you are going into STEM fields Latin is obvious but Chinese, Korean, or Japanese are also useful.