hey there. i haven't been to the gym now since i started my new job, but i was a regular 7 days a week, 2hrs a day guy before that. in my experience you can do pretty much whatever you want. i would definitely take it easy though-i waited about 6 months after surgery to go back.
for aerobics i usually stuck with the stepper, 30-40mins, but i occasionally hit the recumbant bike. stepper gets your heartrate up fast and keeps it there. no running for this guy, i found it awkward having the bag bounce around.....but i never shyed away from a very quick walk on the treadmill at the highest incline i could get...one can still get into the 80-90% heart rate range doing this, but it takes longer. i didn't like the stationary bikes just because it sorta squeezed my flange area a bit, but that'll depend on where yours is placed.
as for weight training, i found if i used free weights, it was too much for my core area and the muscles were actually pinching the stoma, making output difficult, causing cramping. it took me a while to figure that one out. but as long as i stuck with machines, i had no problems and could lift/push/whathaveyou as much weight as my muscles could handle, i was doing ~440lbs on the leg press and was in the 150-200lbs range on pretty much everything else.
and you're not the only guy doing yoga, i do some every morning, it's the first thing i do after emptying the bag :) i'd be careful with that if you've still got stitches in, but doesn't sound like you do
oh, and one last bit of advice :) if you're worried about your core, hire a trainer for a session. explain the situation and tell them you don't want to work your abs too much-what exercises should and shouldn't you de, what machines to use and stay away from. they've all seen it before and will set you up, that's what i did. most gyms (around here at least) offer one free session, but they don't advertise that, you have to ask..THEN they'll do it lol
Dec 2006: Proctocolectomy/permanent ileostomy
Since 1991: Indeterminate crohn's/ulserative colitis