If you are traveling on SN and you have access to *A lounges (traveling in Business or by way of *G status), you can visit The Loft, i.e. SN's lounge, which is very good.

However, with a 6 hour layover, I would consider a quick trip into town to have a beer. Getting to the center on the train takes about 30 minutes, and it beats anything any lounge has to offer.

Also I assume you will be arriving on UA: again if you are traveling in a premium cabin or have status you might be able to visit the Sheraton hotel outside the airport to take a shower.