Muck around
Definition 1 of 2
Pro Tip 1/3Wasting Time
Implies unproductive usage of time when more important tasks await. 
Stop mucking around and finish your homework!
Pro Tip 2/3Playful Context
Often used for harmless, playful activities with no specific goal. 
The kids are just mucking around in the backyard.
Pro Tip 3/3Colloquial Tone
Appropriate in casual conversations rather than formal or professional settings. 
He's always mucking around and making us laugh.