I have a 7 Hour Layover in Seoul. Do I have enough time to do anything? Based on my research, I should be able to clear customs in 90min and need to be at the airport 2.5 hours early to come home. That basically leaves me 3 hours.

We flew through customs when I visited. Literally plane door to hotel bus in 15 minutes. I would think you have time to do something, however it takes about an hour to get into Seoul from Incheon, IIRC. I'm not sure about things to do in Incheon as opposed to going all the way into Seoul.

The train takes 43 mins, so it sound like I might have some time.

If nothing else, there is a Korean craft area in incheon that is free and has traditional Korean crafts you can make. If you have time go to nam de mun

7 hour layover is going to be tough. Kind of too long to just hang out at the airport although there are some lounge areas and enough food choices (besides KFC and Pizza Hut). There are also showers available and I think there was even a place to get a massage. For some strange reason, I couldn't find any ATMs on the airside. But, luckily I had cash on me so I was able to change USD or AUD for KRW. It's about 1000 Won to the Dollar so the prices will sound funny. 4,800 KRW for a Heineken.

As stated, it takes about an hour to get from Incheon to Seoul. I stayed in Gangnam-gu (in 2011 before anybody knew that it was a style). On our trip back to the airport, there were heavy rains and the train was not running. It took about an hour and a half to get to the airport.