How can I take a break in Destiny 2?

Destiny 2 does not have any method to pause the game during any activity. This is due in part to all activities having the potential to be multiplayer through teammates joining your fireteam, with the exception of a few solo activities.

For short pauses, you can simply allow your character to remain dead as there is no penalty for death in most single player activities.

Additionally, if you are inactive for too long (around 5 minute of no keyboard input), the game will kick you to orbit. Remaining AFK in the orbit screen will have the game send you back to the character select screen.