1. Jacob's Well (Wimberley)
Jacob's Well is an obvious first choice. Several amateur cave divers have actually died here due to running out of oxygen while exploring the 200+ foot deep abyss, and others have met their demise by misjudging the trajectory of their leap from the towering rocks above. However, if you don't do either of these ill-planned stunts and simply swim in the well, you'll be just fine.
2. "El Capitan" at the Guadalupe Mountains (Salt Flat)
If you choose to hike to the top and gaze out over the peak nicknamed "El Capitan," you'd better be careful! It's a very steep and narrow ledge, but the view is absolutely worth it.
3. Village Creek State Park (Lumberton)
Be sure to come decked out in hiking gear if you visit this park! That's right - boots, long pants...the whole nine yards. This swampy area of East Texas is notorious for poisonous snakes that are easily disguised by the abundance of leaves covering the ground. It's tempting to wear shorts because of the oppressive humidity, but I'd rather be hot than rushing to the hospital for antivenom.
4. Texas-Mexico border (near El Paso)
I'd probably avoid the border for reasons other than this, but this area of West Texas is known for the infectious Kissing Bug, a name that makes it appear friendly when it's exactly the opposite. Kissing Bugs transmit parasites by biting their host and then dropping feces on the wound. The parasite is deadly in almost all cases, causing heart abnormalities and enlarged organs. Nearly two-thirds of the bugs carry the parasite, and due to the high traffic of the border, transmission is extremely easy.
5. Barton Creek Greenbelt (Austin)
The Greenbelt is an extremely popular hiking and swimming spot in Austin - no surprise there; it's absolutely beautiful. You should be fine if you're just hiking as long as you watch your step, but use extreme caution if you decide to come on your bike. The ground is uneven, there are scraggly tree roots everywhere, and the path is only a few feet across before a steep ledge drops off over 100 feet.
6. Pedernales Falls State Park (Johnson City)
If the hill country is in a drought (which it often is), then you should be fine climbing all over the rocks down by the river. However, if it's roaring and raging, the stone gets super slippery - and losing your footing could seriously send you into a battle for your life. I speak from personal experience: my sister and I almost fell into the water and down a very steep waterfall with rocks at the bottom after (stupidly, I'll admit) jumping over a gap between two rocks in order to access more rocks to climb on. Just use your best judgment (hopefully it's better than mine).
7. Hamilton Pool (Dripping Springs)
Hamilton Pool is arguably the most beautiful place in Texas, so I definitely encourage you to go - but not without reading these tips first. There's a waterfall that trickles down onto a large limestone rock that kids love to climb on and have their parents take pictures. It truly is the perfect scene, but we all know how slippery limestone is. If it weren't for my dad's iron grip, I would've slid off the rock and hit a bunch of smaller rocks surrounding it. Secondly, there are coral snakes here. Don't waste your time trying to figure out if it's red and black or red and yellow - just get the heck out!
8. San Marcos River
If you decide to tube this river, make sure you stay in your inner tube the entire time. Unlike the Comal, this river actually has a current and is full of underwater plants across the entire diameter, not just on the sides. My sister was swimming underwater once and got her foot caught in a plant that she had to disconnect herself from with her eyes closed (she wears contacts). It's a fun float, especially since you don't have to paddle as much, but be careful!