so it would be harder to get your money's worth

The Paris Visite pass is a day pass that you purchase for travel within a specific number of zones. That's what I used and was happy with it. It kind of depends on how many rides you plan to take each day though. The Visite pass is about the cost of 5 single tickets if you buy a carnet of 10, so if you're going to be taking fewer rides on certain days, you could just buy the carnet of 10 for about 15 euros.

Just buy when you get there.