A few places with bright murals spring to mind - particularly one just north of sixth street, round the corner from the hard rock cafe. Big, colourful mural that I've used a couple of times.
Also, some of the lots on congress, across from the Frost Tower (or near by, on the other side of congress) have very peeling, broken up textures that might work well (I'm thinking of the NW corner of 4th and congress I think)
There's also stuff up near the Austin Museum of Modern Art - the building across the street has fairly interesting modern architecture columns, tiles, reflections etc) Just down congress from that is the Hickory Cafe, which again has interesting murals along the wall leading away from congress.
Used the 6th street mural for this background (doesn't show you it but gives an idea of the colour)
Using the wall at the Hickory Cafe
Or you could probably just pose them around the side of the capitol building
(okay, I admit it, this was taken in Italy, but a wide angle lens and a bit of cross processing gives a fairly 'urban' look to a very generic location)
There are probably interesting things you could do with the Austin Power Plant as the
background - either shooting right in front of it on 1st street, or shooting from across the street on the pedestrian bridge (beside Lamar - it has a spiral ramp to the top -
good 'industrial' background there, due to the austin power plant and capitol building.
Shoot it towards sunset for warm light in their faces, and possibly the Austin Power sign
lit up in red and a warm glow on the capitol dome/ frost tower etc.