I hiked the River Trail, the Dragonfly/Meadow Trails, and the Cliffhanger Trail. There was one short steepish bit where you hike down off the bluff to cross a little creek and go back up on the other side, but nothing super difficult (you walk up & down, you don't climb or crawl or anything). Otherwise, it was mostly levelish, running along the top of the bank above the river. It was a real pleasure to be accompanied by the sound of running water and birds for almost the whole hike, and there's lots of shade. It's one of the more consistently shady trails in the Greater Austin Area, I think, along with Northeast Metro, Turkey Creek, and Wild Basin, though I need to try it again in Jul/Aug to be sure. In Feb, it was defnitely pleasantly umbrageous.
The amount of solitude was what I think of as just right--there were enough people around to feel safe, but not so many that they interfered with immersing yourself in the greenness. It is located alongside a subdivision, though, so you're not going to confuse the trail with an unspoiled wilderness. Still, between the river and the plants, it felt reasonably natureful.
Also, there is an interesting little alley on the Dragonfly trail where someone has planted a variety of labeled native[?] succulents--pirckly pears, agaves, yuccas, etc. If you're planning on doing some xeriscaping, this might give you some ideas. I was there the Sunday after the big Valentine's Week Freeze of 2021, and the Agave univittata held up beautifully, FWIW.
I parked at the Two Bridges Trailhead, but next time, I would probably use the Lost Pines Recreational Trails trailhead, which is in a nice park with a canoe launch on the eastern end of the trail. The lot is bigger and easier to navigate, it's a prettier spot, and you start at one end instead of in the middle.
Minor warning: the trail seems to have recently changed hands from the Pines & Prairies Land Trust to Bastrop Co, so some of the signs and names seem a little inconsistent/out of date.
