My thoughts:
The average renter wouldn't know a vintage bike if it bit him. That being the case, you need vintage-style bikes, not vintage bikes. That way, they can all be the same type, brand, etc. It might also help if they were light, rather than heavy.
Your customers might equally like to rent surreys or other non-vintage bikes.
If you ride much, you build up your legs, and can tool around pretty good on a single-speed. But if you have ANY hills at all, people that rent bikes are going to have trouble getting around on a single-speed. I remember last year on the "Tweed Ride" here in Dallas, i saw some people walking bikes up a hill where I wouldn't have even noticed there WAS a hill.
Having seats that you can raise and lower without tools would be advantageous.
On the sleds, I'm not so sure. We used to live near Rocky Mountain National Park up in Colorado, and took the kids sledding some. What worked best for them was the saucer sleds, and you can buy those for like $8, or could then. I don't think I've ever seen snow conditions that would allow one of those Flexible-Flyer sleds to operate. And it was just hard to find good sledding areas, period. Cross-country-ski and snowshoe rental would be other items to consider, but those require a little expertise to use and to rent.