I love your ideas and putting those logs to use! My kids have always enjoyed walking along and jumping from log to log. I also love that you're incorporating your kids into the designing and building process. I think one of the reasons my kids love their playground so much is because they helped with it, and have the ability to modify it, too.
(If you haven't already stumbled across it, you cans see pictures of my kids' ever-evolving playground:
Permaculture Playground and Diner. We've just slowly added to it over the years. We've used a mix of natural materials and pre-fab playground equipment that we were given or were free on the side of the road. I honestly think it's perfectly fine to utilize some non-natural stuff to make it even more fun for the kids....especially when the things would otherwise be in a landfill!)
One fun use for the logs is to make a teeter-totter out of a log and a carved log for its balancing point. My kids really like walking down it, and they've incorporated it into their obstacle course. My kids love playing "the ground is lava" and trying to go from one equipment to another without touching the ground. It's a pretty easy thing to facilitate, too, if you have logs and rounds to walk and jump on!