In reply to Will Rupp:
For rock routes ( mostly ) you could take the path up to the Midi mid station area where there are lots of great bivvy spots. Probably two to three hours with bivvy kit and food.There is a hut but I'm guessing you'd want to save money. Should be snow / water readily accessible.
Walking up to Montenvers is quite doable, probably a couple of hours with bivvy kit. From there you can access routes below and above the Envers hut which are maybe another couple of hours. Lots of bivvy sites around the Couvercle, again probably a couple of hours further from Montenvers.
There are some great multi pitch sport rock routes in the valley such as at Barberine.
Another option may be to buy one return uplift pass to, for example, the Torino or Cosmique areas where there are great bivvy spots and then stay there for several days. Torino area is especially suitable for this, lots of great discrete camping spots.