Empty it out completely.
Consider having an epoxy floor coating professionally installed with a fun pattern.
Don't put in rugs- they can be tripped over. Big rugs are heavy and awkward to store.
Bikes and collapsible things can be hung up when the cars are in the garage.
Use comfortable folding chairs instead of bulky beanbag chairs that adults find uncomfortable.
Add good lighting that isn't fluorescent tubes. Fluorescent tubes do not signal high end hangout room, they signal old garage.
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Wall color
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