I would go with a high quality artificial turf; not the roll stuff at the big box stores. Our neighbors had top quality turf laid in their whole backyard and it really did look great. No maintenance and will add much needed green to your yard. I wouldn't do river rock as you couldn't easily put your lounge chairs on it and it is harder to clean out than one would think even with underlayment. We have over 48 tons of rock in our yard of various size so it isn't that I don't like rock. It still gets weeds, bugs, and isn't easy to walk on. It looks like you need to preserve as much flat surface around your pool as possible. Trying to add or match concrete or pavers will make the yard too quilt-like IMO.