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Okay, right now I play bassoon. My BD told me there were no parts for bassoon for Marching Band so he wants me to learn another instrument over the summer. Any ideas on which would be a good choice?
Right now Im leaning towards trombone because it reads bass clef and my baritone playing brother can help me out, but my BD said either flute or clarinet would also be a good choice.

I'd say to go with trombone, as that would be easier than transitioning to treble clef for flute/clarinet. And having more people on the low brass line doesn't hurt.

A random note: all the bassoonists (even the guys) at my high school are colorguard members. No idea why, but that's how they roll. sweatdrop Maybe if waving around long sticks appeals to you, and your band has a colorguard and needs some more good people, you could try your hand at it.

I'd go with sax.

Alto or tenor, whichever you want.
Sax or clarinet.

Because the embouchure and fingerings wouldn't be completely different from bassoon, making it an easier transition. I think getting used to treble cleff would be much easier than having to learn a completely different embouchure and using a slide instead of fingerings.

But if you have your heart set on trombone, go for it.

The only bassoon player I ever knew would play flute and piccolo in marching band, so I would suggest that. But from personal experience (as a horn player who picked up flute on the side for fun) I would have to say you'll enjoy trombone more. Brass instruments are way funner. It might take you a while to get your lip strong, though. I think it would be worth it. So my suggestion is, learn trombone, and stick with it even if it seems tough. Trust me, by next year, you'll be like an original trombone player.