Moon about
Definition 1 of 1
Pro Tip 1/3Daydreaming Focus
Use 'moon about' when you mean someone is distracted by their thoughts and not focused on their surroundings. 
Alex was mooning about in class again, probably thinking of the weekend.
Pro Tip 2/3Lighthearted Tone
'Moon about' often has a lighthearted, slightly whimsical tone; it's not usually used for serious situations. 
Mia spent the afternoon mooning about the park, enjoying the sunshine and fresh air.
Pro Tip 3/3Romantic Connotation
'Moon about' can imply that someone's aimless behavior is due to romantic thoughts or feelings. 
Since his first date with Julian, Ricardo's been mooning about with a smile on his face.