This happens a lot at our house. The kids never seem top get bored with it. We just keep running out of chalk. I had a delve through my photos and found pics from a few different occasions that we'd done this to post.
All you need to do is wet the floor outside (we have a concrete yard, which is great for this) and then give the kids some sidewalk chalk and some brushes. The colours are so vibrant when the chalk is wet that the kids just love it and it washes off with a quick hose down so easily.It's a good way to teach colour mixing too. Letting them create blobs of colour next to each other and then using the brush to mix them on the floor.
We are super classy here and my kids love to splash about and paint each other with watercolour blocks in an under the bed storage box. I did briefly entertain the idea of getting an inflatable paddling pool until I realised that it would last approximately five minutes after the two hours that I would spend inflating it, and they are more than happy with the storage boxes anyway (we actually had eight kids playing in and around two of these boxes full of water today with the chalk).MaryAnne of Thrifty craft mama posted about using these storage boxes with the lid as a dinky and thrifty sand pit, so we got ourselves set up for the summer with one of those this weekend too. It's great! I chucked in a load of treasure (beads) and a couple of seives and the kids are happy as Larry. Hopefully the lid will prevent the local cats from leaving us their little bum trinkets. I just have to remember to put the lid on it now. This I suspect is the weak link in the plan.