So, when a tree or building is available, you could lean your bike against it, but you should engage one of the brakes so the bike doesn't roll.
What are some good tips about keeping your bike off the ground, when you're not straddling it? In a previous life as an Army medic, it was common to use "blousing rubbers" on your military pants. You would "blouse" your pant cuff with these short elastic bands with tiny hooks on their ends, so that the pants look like they're tucked in your boots. The point being something like elastic bands to press the rear brake lever towards the handlebar?
What do you use to anchor the (front or back?) brake lever ... and are there any other tips for propping bikes without kickstands?