I have done it on foot a few times and it's great not having to worry about a vehicle when you want a drink. I also love the horse drawn trams. Saying that having a bike or car is also great for exploring but being on foot does not take away any of the fun of the TT. There's such a lot of different modes of transport out there horse team, electric railway, bus, steam train, taxi if it's a late one at the bushys tent lol, we had a savers card that allowed you on all forms of transportation last time we done it on foot not sure if this is still available and it was really good, you get to see the island in a relaxed way.