Checklist of things to do before flying to Korea

I am officially three months out from my first trip to Korea. I am wondering if any of you have any suggestions of what I need to make sure I do before flying out.

I'm thinking more like a checklist of "make sure you do this or that a month week or day" or "make sure that you buy this for that " before you leave.

For example, I know that I need to look into you what is required for me to bring ( is a US passport sufficient ) or fill out online for arriving in airport at Seoul.

Any suggestions or recommendations would be appreciated. I want to be sure that I don't miss anything when planning for this trip.

Thanks in advance.