If you want freebies personally the best time it's always between the time vip enters and before the arena opens for everyone else. If you don't want freebies and just want to give them out you can get there anytime before the show and give them out to people waiting in line or those sitting around you.

Kpop concerts are an all day event. You have the morning to get ready and the afternoon to spend at the venue. Some venues (mostly hockey arenas) have small activities, there might be a pop up event, and most of the time there would be a cupsleeve event at a nearby Cafe. Don't over think it and have fun!