a) pilates. You can get a DVD from Target, and I like that windsor pilates series. Lots of floor work. VERY good for back and belly strengthening.
b) I have so many slings, I can't begin to tell you. I tried a mei tai, a custom made native sling, a mayan metal loop sling, even the full wrap kind made with jersey (I seriously do not recommend that one since it stretches like mad while you are wearing it.)
What do I like most? a very basic native sling, it's sized medium (I am a 12/14). The other one I had custom made was ridiculously big. I wish I had known to just go try one on first or knew how to sew so I could have remedied the first situation, but the second native sling was actually much better quality than the first.
here is a pic of it in use on a 1+ mile hike: http://farm2.static.flickr.com/1416/1198490841_95ffecfbd7...
note how the sling is adjusted for a toddler to distribute weight over the shoulder. It's actually folded into a sort of hip seat for my daughter.
http://farm2.static.flickr.com/1215/1199332342_eb4544a224...
not the best view of the sling itself, but the subtle fold in front is the key to comfortable toddler wearing.
I don't have any pics of this sling in action as an infant or using it to breastfeed or shop, but I was able to do all of these. You can adjust the sling in a lot of ways. The looser it is, the more you can move that sucker around to the back, or on the hip. If the kid's bottom is in the "pocket", and the sling is adjusted to your own comfort zone (practice with the kid in the sling), you will have near full mobility.
The only thing I didn't like on that hike was the fact that my left hip took a beating with the weight carried mainly on that side. She got tired early on and it just wiped me out to carry her the rest of the way. It would be worse now, she gained a couple of pounds since last August. We recently got a kelty for hiking seriously, but the sling is still awesome for dog park, neighborhood walks, farmer's market, and even a quick blanket for the car.
If you are in Sacramento, you can stop by MoonFaeries, the MSN store, where SAHM's make the slings and you can try them on and someone with knowledge about the fit can help you figure it all out. That's where I bought mine 2+ years ago, and it's the best carrier I've owned.
I also just checked out the link for the ergo carrier, and that looks rad for light hiking and shopping too. I now covet that. 92 bucks seems steep though since that sling I bought in Sacramento was only about $20 and supported a local mom.