Saxophone. Baritone sax is a great way to express yourself through bass. Bass guitar is nice and all, but with a barry, its a completely different experience. You can put alot more feeling and get a full body experience of the bass. It hits the spot alright. Tenor is good for just general melodic lines, and you could record yourself playing different parts on the piano and guitar, and play the sax along to it. Become your own band.
Trumpet is good for a challenge. It puts a lot of pressure on your brain though, so most people reccommend not playing it for TOO many years. I think 10 years is basically the maximum most people would reccommend.
Flute is cool. Its great for coordination, because you can't look at your hands, unless its in the mirror, which is pretty confusing also.
Trombone and violin are always a challenge, and really quite different to guitar, when you are physically playing. Getting a nice timbre out of these instruments (particularly violin) is a very rewarding achievement, as they are renowned for their considerably horrible sound when you first start out. Getting tidy, sharp, quick notes on a trombone is always admirable.