Is it alright to play sports like volleyball and baseball with a nail left inside my wrist? (scaphoid fracture)my doctor says it need not be taken out.

Scaphoid fractures. can be treated with a screw that is commonly "buried' in the bone so that after it is healed and the OK is officially given ( that is VERY IMPORTANT ) one can use the wrsit and the hand normally. These screws are generally not taken out though sometimes screws can shift and fractures can settle a little but that is the exception. ANY SCAPHOID FRACTURE MUST BE OK'd by treatingDOC to resume activity.
If the fracture. Has completely healed, yes. If you develop any symptoms with your activity, you should probably stop and ask your doctor for specific recommendation.