The Roissy Bus could not be more convenient and inexpensive. If you only have one manageable bag, you should do it. The Roissy Bus is 10 euros per person (buying tix inside/US credit cards accepted)

As for the easy walking directions to the hotel -- once you exit the Bus, just turn left and walk 20 meters to the corner -- cross the street, and then turn left and cross the street -- keep walking til you see the Benetton store, pass the store going straight walking one short block to the end of the Benetton store (it takes up a whole block) and turn right -- Rue de la paix -- go about 300 meters and the Park Hyatt will be on your right in the middle of the block. 5 minute walk at most and you get the lay of the land to boot.