The Loft Hostel (70 rue Julien Lacroix), and it was clean, modern and included breakfast. Moreover, if you are going in a group you can get your own room with 4 bunk beds and its own private bathroom. The main downside was that they provide a towel the size of a hand towel... so knowing this, I suggest you bring your own. It is located in the neighborhood of Belleville (where Edith Piaf was born), and nowadays is quite hip. There were several bars with an alternative crowd in the streets surrounding the hostel, and some very cool graffittis to admire. By the way, if you are into it, there are street art tours whose meeting point is actually the Loft hostel. From the hostel you can take the metro to Châtelet, and from there you can transfer to the main lines, or else you can start walking.
travel by bike, using a system recently put in place by the city (where you can take a bike at one point, ride it to your destination, and drop it off there). And if you really must take a bus, by all means use the public transport: for the fans of Julio Cortazar, line 63 takes you to some emblematic places like the Jardin des Plantes, Sèvres-Babylone and Saint Germain des Près. You can buy sets of 10 tickets (that are good for metro, bus, RER and tram) or a card for 1,2,3, 4 or 5 days. More info on the public transport system here.
spend all your money on books and vintage suitcases. Oh and I could not end this post without recommending Glaces Berthillon, possibly the best ice-cream in Paris. You can truly taste the natural, high quality, seasonal ingredients, and it is rather dense, rich and creamy. Their original shop is located in Île de Saint-Louis (29-31 Rue Saint Louis en l'île) and it is a small café with that grandma feel that I totally love. Their pastries looked scrumptious as well. Oh Paris, how I miss you.