Leaves on bushes/hanging down from trees. Master Sang H. Kim demonstrates/advocates this in Ultimate Kicking Drills and the Korean team in "Art of Competition". It's a great way of improving accuracy, you pick a different leaf for each kick and it gets you used to different ranges/heights and movement between ranges. I'd personally advocate that over a tennis ball (as the tennis ball you have to reset/wait), but it may vary on your environment whether it's too cold to kick about outside.
I wouldn't advocate whacking away at branches for your own safety and there's no point in causing major damage to the tree/bush, but the leaves will likely fall off in the autumn anyway and kicking the odd leaf off won't do any tangible harm to the tree (not that they have pain sensors or brains).