You can use these Google+ features as Feeds content sources:
Username: You can pull in a public users public posts by connecting their profile URL. You'll see accounts with formats like https://plus.google.com/106766146657870989685 or https://plus.google.com/+RyanMacInnes, you'd give us 106766146657870989685 or +RyanMacInnes, respectively.
Google Places: You can also pull in the reviews of a location from Google Places. Use this tool to find out what the place_id is for the location. Google will only allow you to pull in a few reviews at a time, but they should build up over time if you let it go for a while.
Hashtag: Simply use the hashtag you want to pull in after clicking the GooglePlus logo, then clicking the "hashtag" tab at the top of the overlay. Feeds will pull in any public posts it can for this source.