What are good exercises to do after prolonged bed rest to get lightheadedness to subside?

Slowly get active: I would start by spending time sitting in a chair. If you tolerate that fine, start to walk. When you first stand hold onto something and wait 20 to 30 seconds to see that you are not lightheaded. Use a can if necessary. Start slow, pick an easy goal, and each day try to extend how far you go. Make sure you are well hydrated and eating healthy. You may need physical therapy.

Gradually: increase activities under supervision. Start with just sitting up on the side of the bed for increasing periods of time. Activate and passive ROM exercises can begin before even attempting to sit up. Autonomic failure can lead to fainting so ambulate with assistance at first. Avoid prolonged standing in one place for several weeks to months (standing in line)