Use Bicycles

In Paris, they have these pay-as-you-go bicycle rack things. They're everywhere in the city, and when you borrow from one bike rack, you can return to the bike to any other bike rack you might come across. Our tour guide recommended them to us, and it made me wish we hadn't bought metro tickets. They're on a timer thing, so you pay for however long you use them, be that an hour or eighteen hours, but they aren't super pricey, especially if you're returning them to a bike rack as soon as you're done with them. If you aren't having to lug suitcases around, make use of the bikes.