The tenant should be allowed some freedom when using the outside property - but within reason.
No, they cannot change the landscape without your approval. Yes, they can work on their cars in the driveway. If they cause damage to anything outside by something of their own accord, then yes they are to be held responsible just the same as inside. ie: tenant decides to pull all the bushes up because they don't like them, they are responsible for paying to replace all of them.
The tenant's responsibilities for yard maintenance should be outlined clearly in the lease. ie: mowing the lawn every 2 weeks, raking leaves, etc.
You are held liable for injuries that happen on the property if you're found to be negligent. ie: Your steps are notoriously slippery in the rain, tenant brings it up and a concern you dismiss it, tenant falls and breaks something. That's on you. If you fixed it and they still fell, that's a grey area but most likely on them.
Things that damage the property like paintball shouldn't be allowed because of the possibility of something on the house being damaged, that you can stop. Plus it's a liability.
One note, you need to outline these types of things in your lease some how.