Developers should be writing code in the current sprint.
QA should be validating the code completed in the last sprint and/or bug fixes found in the current sprint.
Product Management should be finalizing/confirming requirements for the upcoming sprint and answering questions for this sprint.
Only take on what you can do “-1” – the “-1” being something is always going to go wrong, so cramming in your delivery to your topmost capacity is a surefire way to not hit your upcoming goals. Give yourself time for customer tickets, unplanned meetings and the unknown.
The goal of any sprint is to do what you said you’d do.
And that’s how you plan a sprint.