Rocks - I hate cleaning mud from everything. You will find mud months later when doing something under your Jeep.
River or Beach? I have been to the beach at Pismo Beach state park in California few times prior to our move to Texas.
I don't like salt water, as it is a corrosion contributor and will speed up any corrosion/rust on your Jeep. The sand is also very fine and can get into bearings and seals where it is very abrasive if not cleaned out quickly.
So, I guess a river would be preferred.
I like running trails up and down mountains and in the high desert. I agree with Todd on mud, it's fun a couple times but cleaning it all out gets old. Never done a beach, and rivers you can wheel through are pretty much nonexistent in my area.
Rocks - Being out here in the desert that is mostly what we wheel in. They give you a fun challenge trying to find the best wheel placement to overcome the obstacles. I try to stay away from mud as much as possible.
Rocks, rocks, rocks, and more rocks. I hate mud! But here in Ohio there is a lot of it inbetween rock sections so it is something that I just have to deal with. I do try to avoid it as much as possible though. River vs. Beach, river wins. We wheel a lot of old creek/river beds. Theyre also fun to float down with a cooler full of beer on a hot day.
Living in Utah it’s easiest to say “Rocks” with Moab only 3 hours away it’s not hard to do day trips on a regular basis, but “Rocks” feel like it needs to be quantified. There’s boulders, slickrock, granite slabs, and more.
If I had to pick my favorite off reading i’d say “Climbing” and “heights” I love to feel my Jeep climb up incredibly steep terrain and love to stand on the edge of a cliff that no other vehicle could have delivered me to. I love looking at a field of boulders knowing that I can climb over them while others struggle to get through even on foot.