Public transport is cheap and reliable with a Navigo card. Depending on how long you are planning to stay in Paris, this could impact which type of card you buy: monthly or yearly.

"Don’t bother with a bike - the driving is too nuts and not enough places to park them after use"

Paris is pretty small, so I would highly recommend walking as much as you can. It's great for improving your navigation and directions around the city, and also means you will see so much more than being on the metro.