Your outdoor shower needs a floor, and that floor should be constructed with a non-skid surface. You can simply lay pavers, or you can build a small deck-type floor made of weather-resistant wood. Some people use gravel for the outdoor shower floor, but that doesn't sound very comfortable -- and gravel tends to travel!
Another option is a concrete floor, which you'll build by first digging out a square or rectangle in the earth and framing that with a wood border to hold the concrete while it sets. Pour a bed of gravel on the bottom, level it, and then pour concrete over the gravel up to the tops of the wood border. To help avoid standing water, be sure to tilt the border wood frame slightly toward where you want the water to run off.
You can create an attractive, non-skid concrete surface by giving your concrete a broom finish. When the concrete is partially hardened, use broom bristles to give the surface a swirly pattern. If you've set the concrete at a slope, be sure your broom marks run toward that edge to help with drainage.