Many times I just throw them in the pool or hose them down to clean up the mess. See, told you it was easy! Plus I always use washable fingerpaints to make the clean up easier.
I usually just take white craft paper and thumbtack it directly to the fence boards. Easiest way I've figured out so far. Here I've just taken easel paper and unrolled it and we also make sure to paint on both sides of the paper to make it stretch as far as possible.
I've also recently discovered that solid wrapping paper makes great painting paper too. Black is awesome, it really makes the colors pop on the paper. You can easily find solid wrapping paper on sale (think after holiday sales).
I have the kids use paintbrushes, foam stamps, their fingers, whatever they like. Dollar stores and dollar bins are good places to find random painting supplies. I even take cheap kitchen sponges and cut them up into shapes sometimes.
For paint pans, I just use those cheap aluminum baking pans. They hold lots of paint and are easy to rinse out and reuse.