Take a hike in the Blue Mountains
The Blue Mountains are a small mountain range to the east and southeast of Walla Walla and include the Umatilla National Forest. This range isn't large or high (peaks are only around 6,000 feet) like the Cascades to the west or the Rockies further east, but it is a scenic range nonetheless with some pretty hikes with some sweeping views and trails through meadows and old growth forests.
From Walla Walla, there are two primary ways to get to the hiking trails. Drive east along Mill Creek to the end of the road to the trailhead. Alternatively, drive south to Oregon and exit near Weston, go to Tollgate, drive south for 8 miles, take the Summit Road to the trailhead.
From there, the elevation change is minimal, so it is more like a beautiful walk in the park. If you want to make it more strenuous, drive south into Oregon to Gibbon (lower elevation than Summit Road). The trailhead is 1900 feet lower and you can hike up to the upper trailhead on Summit.
We chose to be lazy and went through Tollgate. We found a nice trail and hiked for 2 hours (about 6 miles).