Until Southwest gets their act together with Hawaii, taking them to the islands only really makes sense if you live in the Western States or are doing an extended stay in California before heading over.
Right now they are flying only from San Jose and Oakland, with more airports to follow. Going over, since you can do it all in one day, you can grab a flight as long as you connect through SJC or OAK. But coming back, you have to do a multi-city run.
I just ran some flight searches and they would let me book a one-way flight from BWI to HNL on November 12th for $312 (BWI-AUS-OAK-HNL). However, for a return trip on November 21st, they were no flights. I had to do a multi-city trip which gave me HNL-SJC on 11/21, and then SJC-BWI on 11/22, for $517. You could do a better run on Hawaiian, for example, for $788 (BWI-BOS-HNL; HNL (redeye)-BOS-BWI arrive 11/22).
Eventually Southwest will have it worked out. They plan on offering service here in Sacramento at some point. But it's almost an afterthought for my wife and I because we can take Hawaiian direct to HNL or Alaska direct to Maui.