For your solo work, there is no reg' that says you cannot just go up and do pretty much nothing. No practicing, etc., just burn avgas. However, you are gonna have a checkride coming at some point, and the best use of your flight time and dollars is gonna be to practice for that practical while soloing. I imagine all of us had that day or two when we'd just go up solo on a beautiful day, for the heck of it. We do it after we get the Private, too. But you really need to be working on your manuevers plus basic pilot skills [altitude and heading control, airspeed/power/etc. control] so that you demonstrate to the DE that you are ready to be turned loose with a new certificate in your pocket. Get with your CFI and come up with a program for solo work, you should have had this already. He or she will certainly know the areas you might want to concentrate on. Like Ron and the others said, the CFI should be overseeing this process. And if you find that there is some maneuver you aren't able to do very well [and you should know by now the parameters within which you must be able to complete each one on the exam], then discuss these with the CFI and either have another dual flight to go over them or figure out what to do yourself to fix them.
