Oh boy, I would have a field day with your yard (and your wallet). It's a big space and well landscaped. Do you have landscape lighting now ? Do you have a budget for the audio improvements ?
You have two options on the audio side as I see it:
1) you can stay with conventional speakers - you could start with two but I'm thinking four would give better coverage for the pool area, and use a connect AMP to drive them. If you wire them in parallel you may need an impedance matching device depending on what speakers you go with - two 8ohm speakers wired in parallel will ne 4ohms (fine) - two 6ohm speakers wired in parallel will net 3 ohms (problem for the connect amp).
2) Though I'm thinking a 70volt system would be perfect. You could pump a Sonos connect into the 70v amp as a source. In 70v the speakers are basically daisy chained and theirs no consideration for ohm ratings. Example here:
http://www.snapav.com/p-1550-kit-es-ls-sat-6.aspx
In either case I would look at placing the speakers such that they are inside the estate fencing on the far side of the pool facing back towards the house. This will give the best sound reproduction for the pool area while keeping the sound "in" your yard, facing your space. Between the diving board and the slide would be for #1, and just past the slide to the left before the ledge for #2. I'd have to give it a little more thought but I think you;d want another pair to service the seating area by the firepit. Just an FYI, you can split the output from your cable box and put that into your Sonos connect so you can listen to a ball game (TV) outside too.
As for lighting, I would definitely uplight that tree to the right of the walk headed towards the pool, and uplight some of the other shrubbery and trees in the elevated beds. Path lighting outside the pool gate would be a must too. I might consider something like this in your retaining walls, especially out by the fire pit:
http://www.landscapelightingworld.com/H ... -p/668.htm
Just some starter thoughts !