"Let's battle!" is usually the first thing my 5-year-old, Falcon, says in the morning, after "I have to pee" and "I need to eeeeeeeeat" (all e's included). Two-year-old Bear usually says yes, as long as he has his special Black Sword. When Falcon finds his best friend, their hugs morph into tackles and beating. One of them sits on the other while they shriek. Then they get up and chase each other with 2-year-old Bear behind.
Or they take turns (this actually happened) hitting each other in the head with foam lightsabers.