A solar shower is a fun and highly efficient device for summer. It operates by attaching a standard garden hose to the shower — water is heated inside an integrated solar-thermal tank. After 1-2 hours the water will be hot enough for a shower. Solar heated shower are also handy for camping — you can find portable solar showers here.
Make sure you use a lead-free garden hose with a solar shower.
There’s an “Outdoor Solar Shower” available at Amazon.