Well, your AVR has a powered Zone 2, so you're set for everything but remote control. An RF remote control for outside would be the simplest and cheapest solution. If you would prefer a mobile device / WiFi solution then Roomie or iRule plus an IP-to-IR gateway like the iTach would be the path. The limitation is that it's difficult to do much more than select sources or adjust volume. An RF remote can be <$100, a Roomie / iRule solution around $150 (plus some learning / programming time!)
You don't mention what source(s) you want to listen to, but the best solution is probably either a Sonos Connect, attached to your AVR and using the Zone2 functions, or a Sonos Connect:Amp that's self contained. The Connect would allow you to utilize any of your AVRs sitting around, too, but the limitation is going to be remote control, again. The Sonos remote app is excellent, but it won't control other gear - so you'd need a separate solution for volume and power on/off. Which is why I'd suggest the Connect:Amp. It's a freestanding solution, but you could certainly connect it to your Onkyo for use inside as well as outside. And that would get you a single-app solution.
But, for the cost of a single Sonos Connect:Amp, you could likely sell your existing AVR(s), buy a new Onkyo NR system that would have much of the same functionality built-in (including the app support and many of the same streaming music services).
Hope that helps,