Hey, saw this post and I thought I'd respond.
To qualify myself, I'm a personal trainer, it's what I do, hehe.
First of all, the evidence suggests that exercise is good for people with Crohn's Disease. The only issue is over exerting yourself. IE exercising intensely 7 days a week, ect. For instance, I just had a minor flare. I'm coming out of it now and getting back into my routine of 6 days of lifting a week. Normally I workout hard for at least an hour. Since I'm just coming back, I've only been doing 30 minutes. As others have said, listen to your body, but at the same time don't be afraid to push yourself a bit!
As far as exercises in general. Anytime you do anything do to with abs, crunches, sit ups, planks, ect...if you're in a flare or even if not, be near a bathroom.
Basically, there's not specific exercises that you shouldn't do with Crohn's, just some that might send you running to the restroom a bit faster. If you've recently had surgery obviously that's an entirely different story.
My advice always is, do what you can. Some days you may only feel up to a walk, others you can pound out an hour lifting session.
I've put together some exercise basics for Crohn's in this video if you'd like to check it out. I hope that helps!!
https://www.youtube.com/watch?v=_9VFd7_mxpE