Here's a few suggestions and random thoughts if you come to Austin for a nursery crawl:
Zilker Park
http://austintexas.gov/departm...
Fairly central to everything, across the street from the botanical garden. Barton Springs pool is right there. Picnic tables, but no covered pavilions. Difficult to plan for food, and then take off to visit nurseries. Who stays behind and watches tables? Walking to lots of restaurants and bike rentals. Possible to park at the botanical garden for $2. and just carpool to nurseries. No food allowed in the botanical garden.
Barton Springs Nursery
http://www.bartonspringsnurser...
Has a little bit of everything. One of the best places to find native plants. Bananas, tropicals, pond plants, cacti, etc. Lots of gorgeous pots, too.
The Great Outdoors
http://gonursery.com/
Huge nursery, recently renovated. On Congress Av. Another nursery that has everything...or can order it for you. Lots of Mondavi stock too. Has a large tropical greenhouse with hard to find ferns and other exotic plants.
The Natural Gardener
http://www.naturalgardeneraust...
Great organic nursery with so much to see and buy. Lots of natives, vegetables and fruit trees, soil mixes, and entertainment.
This one is a little farther out than the rest, but worth the trip.
Whole Foods was a good place for lunch in the heat of the day. Parking is underground, so we left our plants in the cars so they didn't cook.
There are other great Austin nurseries, but they are too spread out for a day trip. Hope this helps. We're still in the middle of living out of boxes while the house is getting some much needed TLC, or I'd offer our place as "base camp."