There is so much to do here. Where I'm stumped is the "relaxing" part. And also many thoughts I have for one night include food so if you're grocery shopping...
Here are some ideas--get your food at the Boise Co-Op--8th & Fort.
If you're in the mood for a walk, head up 8th St. a few miles from the Co-Op till you get to a parking area on your right. Head out for an easy hike in the beautiful Boise foothills. Everything is really green right now and the creek running through Hulls Gulch is running high.
If you want to people-watch, head downtown. Start at 8th & Idaho and wander from there to find the perfect patio to sit and have a drink and take in some of the downtown atmosphere. Grape Escape is great for a glass of wine, Bittercreek Alehouse has a zillion microbrews, the Falcon Tavern (just east of 8th on Bannock) is super mellow. Berryhill is more upscale. There are lots of other choices, too.
If you like live music and an alternative crowd, you might wander over to the Neurolux. I think it's on 11th--maybe 12th--between Main and Idaho.
Other walking ideas include parking by the Library---between 8th & Capitol Blvd at Battery St.--just south of there is the Anne Frank Human Rights Memorial, a free outdoor attraction that is very cool. From there, you can take a walk on the Boise Greenbelt and enjoy the river, trees and wildlife.
The view from the parking garage at 8th & Idaho is cool.
The State Capitol building is worth wandering around--not sure how late it's open.
For sushi--go to Shige Express for happy hour dollar sushi from 4 till 8 or 9. 8th & Idaho--so yummy and affordable.
There's a few ideas for you. Hope that helps!