I was cleared for everything at 6 weeks (but I was walking since post-op day 1). I didn't actually do more than walking until about 8 weeks though...hot yoga, zumba, rock climbing, upgraded my walking to alternate jogging laps. I also did minimal strength training (and still do), but only because I hate it, not because I can't.
These days I run 5km a day and do 15 mins of strength training on some body part or another every day. (Full disclosure, I was off my running routine for about 2 weeks to due illness and laziness, but am back on the wagon as of this morning!)
I have loose skin.
Agree with @catwoman7 as well: loose skin after massive weight loss is pretty much unavoidable except for the lucky few.
Also agree with @Healthy_life2: strength training/lifting helps to tone and fill in a bit, but will not eradicate the loose skin completely.
I will say this from my experience though: I really, really, really hated the look of my loose skin about 15-20 lbs ago, I guess because there was still some fat under there and gave it a rolling look. Now, that I'm leaner, the look is not as bad, to me at least (still hate it, but I'd remove the 3 "really's" now, lol). In my "problem areas", the skin looks more like wrinkled fabric now and is much more easily hidden...so this is less annoying to me.
With that said, I am scheduled for an arm lift and breast lift revision (to address side boob) next week, which was scheduled 6 months ago. I was thinking I wanted a tummy tuck too, but I have since decided that I can live with it as the loose skin is limited to above my belly button (you can see a pic of how I hide this in the "Albums" section in my profile)
...my butt is another story. I have been doing lots of butt exercises but there doesn't seem to be much improvement there. Mr. says it may be because my ratio of running to weight training is too long-distance-running-centric: Weight training plumps the butt, endurance running flattens it. He suggested I add in some short distance sprinting in my runs, which I tried this morning, but I'm not sure I like doing it in and around the neighbourhood, lol...I will try it on the treadmill tomorrow.
Wow, I just re-read this post. Sorry it's so long!