I took the Spicewood Springs trail up to the road, then the connecting trail down to the river, followed the riverside trail well past the "no bicycling" sign and then doubled back through the campgrounds to my car.
The springs and pools near the trailhead were flowing very profusely. Keeping my feet dry was a challenge. Running shoes were probably a bad idea - sandals or waterproof boots would have been less worry.
The trail starts in a lush riverside area, proceeds up past a series of waterfalls and pools, then tops out in a typical hill country oak/cedar woodland (fairly open - no dense thickets or cedar breaks). Finally it moves back down to the riverside area with lots of tall deciduous trees.
The difficulty rating comes from crossing the creek over and over at the beginning. Rocks are slick, the water level was above the stepping stones, and it wouldn't have been too hard to slip and end up taking an impromptu swim (or getting injured, if that's your thing).
Near where the trail starts at the south end of the camping area there are several pools popular with swimmers and sunbathers. Most visitors to the park stay near the river and this area is especially heavily used, hence 4 on the solitude scale. The rest of the trail was completely deserted (and has been completely or nearly completely empty every other time I've visited).
"This book will be in great demand by travelers, park visitors, and aficionados of Texas landscapes."