SSC is a Soft Structured Carrier. Beco and Ergo are both big names. Infantino recently started selling what they call a "Wrap & Tie" and I was impressed w/ the quality. BabyHawk is a brand of Mei Tai (which the Infantino company copied, among other makers of MT carriers). Then there are Ring Slings (RS) and Wraps (Moby Wrap is a brand), and simple pieces of cloth. It's AMAZING, really, the options available. I've done the zoo w/o a stroller before and regret it *only* because I didn't have anything to hold our crap.

Toting the toddler on my back was no problem.
So look into a Mei Tai, a Wrap, or a Soft Structured baby carrier. I carry my 30+ lb 2.5 yr old frequently, as well as my 13 mo old niece. I'd carry my 3 yr old cousin, too, if he'd let me, but he wasn't accustomed to being worn as a baby and isn't a fan.
Here's the MDC forum for babywearing:
http://www.mothering.com/discussions...play.php?f=245
And
http://www.thebabywearer.com has a GREAT selection of the various types of carrier available. It has instructional videos and pictures, too. I don't think there's a carrier type out there that isn't on TBW. It really gives a good variety to look through and see what works for you.
ETA: My 3 yr old cousin is currently in my Ergo on my back. He's sick.

So even if he's not a big fan, he's tolerating it. Poor guy.