You simply need a power amplifier for the 3rd zone outputs on the Denon. Your outdoor speakers need more power than the dining room speakers so the low lowest power would drive these. You could use a small digital power amp such as thishttp://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-380
or any basic power amp, integrated amp, or another receiver if you have one.
Your Denon would control what plays and how loud.