laptop to DAC wireless ... what's the best way?

Unfortunately, the USB port on the Airport Express does not output sound. It only works with USB printers. The 3.5-mm audio/optical jack is the only way to get sound out of he Airport Express.

You can find AE's for ~$30 used. Depending on what DAC you currently own, it might be less expensive to buy an AE and replace your DAC.

Or check out the "NAD DAC 1 Wireless USB Digital-to-Analogue Converter".

There are a lot of options, but the performance is all over the map:

AirPort Express - toslink out
Squeezebox 2,3, Duet and Touch - no longer in production
Sonos

The easiest to use is probably Sonos, but it sounds really bad without reclocking. Go to sonos.com for a demo. Probably the most popular world-wide.

Steve N.
Empirical Audio

If audio only I like the Squeezebox Touch, but beware that USB connectivity is not officially supported and pretty much only DACs that adhere to the USB audio specification will work. In my main system I use a Mac Mini to get video and more flexibility, plus options for playback software, which absolutely makes a difference in sound quality.

Mac Mini > Meicord ethernet > Airport Extreme or Time Machine wireless > Squeezebox Touch with USB converted to output > Your DAC