First: If you can run speaker wire outside easily, and it won't damage the side of your house, or you are adding siding shortly, then wire the heck out it! WIRE! WIRE! WIRE! I would recommend you might want to put a piece of cat-5e or two out there as well so you (at some point) could add a security camera outdoors. The cat-5e can provide power and communication so you would be able to easily utilize it.
For speakers, you typically have at least a stereo pair outside, so pulling wiring for two speakers makes sense, even if you run them as mono from a single output, it would be a good idea to do that. If you have a large backyard, then pulling wiring for three speakers would even be a great idea. It's a unique time when you have the ability to wire a home properly.
The outdoor zone can really be run a lot of different ways, but you really can't use the 'center' channel as a solution. It's not a second zone. Anytime you turn on the receiver it would be playing audio outdoors. It wasn't designed for that, and is the wrong solution.
You can use a bluetooth receiver and a small amplifier to run the outdoor zone as a stand-alone option. You would want a volume control with that as well. You could also get a small stereo receiver to run the outdoor zone. Craigslist often has old receivers that are ideal for such a setup. Easily under $100, maybe under $50 to get the receiver setup for that outdoor area. A couple of outdoor speakers from Monoprice, and some 14 gauge speaker wire from Monoprice as well.
Speaker wire: $25 -
https://www.monoprice.com/Product?p_id=4039
Outdoor speakers: $30-$150 -
http://www.outdoorspeakerdepot.com/patiospeakers.html
Stereo Receiver (new): $95 -
http://www.accessories4less.com/?type=&page=category&action=&id=stereoreceivers&skip_redirect_suffix=&view_id=499a8b4bc09cc679fda5d577028a9caac25177fc&mode=&search_query=&category=&thumb_sort=store_price.asc
Craigslist receiver: $25 -
https://washingtondc.craigslist.org/nva/ele/6202001914.html
Check your local Craigslist to see what is available, but we have a ton in our area at great prices from people just getting 'junk' out of their house, which is awesome for that type of setup.