The phrase "to watch the fun" means to enjoy and observe an entertaining or exciting situation happening without actively participating in it. Full definition
They were told they could stop whenever they wanted, but then their partner would have to stop watching the fun videos and take over. (scienceofrelationships.com)
When the night is over, pull out the sleeping bags and watch a fun movie with the kids. (lovingmomentsbras.com)
Great for snack time and watching the fun unfold. (massaudubon.org)