The Riverwalk just had a major expansion in 2009 called the Museum Reach. They actually built locks to be able to reach the upper portion of the river. Anyways, it's a beautiful walk. There isn't too much shopping on it yet as the tourist traps haven't had the time to proliferate it yet, but I'm sure they will eventually. I highly recommend riding/walking the new portion at dusk. Under each of the bridges, they incorporated some kind of artistic lighting that's different under each bridge. Some of them are really beautiful.
Check out this link for more info:
http://www.sanantonioriverwalk.com/museumreach.html
The picture in the link with the bridge with the fish is amazing at night because all the fish individually light up. The pictures on this site don't do it justice. By the way, if you're going to be spending the day on the riverwalk, the river taxi is really neat. Certain river boats have checkered flags and you just flag them down like a taxi and they'll take you anywhere you want on the river. You pay a set rate for the day and it's good for 24 hours.
Also, the best Tex-Mex, Margaritas and value (where the locals go) is Alamo Cafe. If you go, try to visit the one on Highway 281. The food and drink is the same, but the ambiance and restuarant itself on 281 is MUCH NICER then the one on I-10. Here is the link:
www.alamocafe.com
It's not the most "amazing" mexican food you'll ever have, but the value and service can't be beat. We eat there close to weekly.
For "amazing" Italian, the locals love Paesanos. Its more on the pricey side. Definitely not as much value, but really good Italian. If you go definitely get the Giant Calamari on the appetizer side. My wife and I go there just for that!
http://www.paesanos.com/
For super pricey anniversary style dining, try The Fig Tree. It's downtown on the riverwalk. It's hands down, our favorite restaurant in the world. It's expensive, but if you're celebrating something, it's worth the price.
http://www.figtreerestaurant.com/index.php
Hope that helps! Have a great trip.