Masks were required on the plane and almost everywhere indoors in Korea. We spent time in some of the parks and had a great walk around the Seoul Tower where we did not have to wear masks. So keep an ample supply handy. We took public transportation and the kids were kindly reminded to put on (or pull up their mask) in order to ride, so they are very strict.
I highly recommend the T-money card for using public transportation. You can also use this to pay airport train, taxis and a number of stores and restaurants! There are times the bus was only 100 Won (around 10 cents) and there was also discounts when transferring from metro to bus or bus to bus within 30 minutes. You can get up to 4 transfer discounts per day. The 2nd bus fare was often free if you could swipe your transportation card in under the 30 minutes to the connecting bus which usually wasn’t difficult. Some of the stations were large and took time to walk to the different platforms or outside to catch a connecting bus.
We went to Lotte World Adventure during the week, arriving shortly before they opened at 10am. There was already massive lines of people waiting to get in. The staff were very good in the way they organized the lines as none of the halls were blocked, so even with the massive amount of school groups waiting to get in, the whole process was very orderly.
The lines got noticeably longer as the day went on, so I would suggest getting there early or pay extra for the premium tickets that can ‘cut in front of the lines’. They have their own entrance to the rides, so make sure you don’t line up in one of the expedited lines if you only have the regular ticket…