What's the festival in Suwon? The Hwaseong Festival? To be perfectly honest, that might be interesting for all of five minutes. Otherwise, Suwon is just another Korean city.
Also, two nights in Jinju you might be TERRIBLY bored. I've had many friends go for the lantern festival and said that, other than that, there was nothing to do. The same goes for Gwangju and Mokpo.
Do you or your girlfriend speak Korean? I know that I sound terribly negative against going to these places, but the fact is that a lot of the things that Koreans can do (find certain popular foods or restaurants, go to unique places), foreigners can't because they don't speak Korean/use Korean guides. Of course, you can point at stuff and guess what you're going to get. You can also just go to the busy place and see what happens, BUT I know some people don't like to do all that.
I second nme on the bus. It's EXTREMELY close to the hotel and the bus is waaay cheaper. The Foreheal is actually fairly new and looks fancy haha.
If you're going to Everland, there's a red Seoul city bus (white on the bottom, red on top) that takes you to Everland in 20 minutes. It's #5002. And it's just 2,4000W. The way there takes maybe 30-40 minutes (stops a few times), but on the way back it's barely 25. Tip: On the way back, if there's many people waiting in line, you can stand in the aisle. When the bus looks full (빈자 counter on the front reads 0), just pass the line of waiting people. Otherwise, you'll have to wait roughly 20 minutes for the next bus and that can be annoying if you're tired. The subway takes WAY too long and the Everland bus times aren't as convenient and it's like 12,000W. This red bus runs every 20 minutes.
Did you already book your Japan tickets yet? I was going to suggest look into flying from Busan and saving time and some cash. If you're in Seoul, you can have your hotel use Hanshin Express or similar company to mail your luggage to Busan for literally 20 for two giant bags.
Damyang is a great day trip. Gyeongju, I might suggest staying one night there. That's because getting from bus/train all the way to the temple and grotto can take some time. Like an hour each way. Then with the time there walking around and getting back that's a large portion of your day, even if you start super early. Then, the next day, you can do the museum, downtown attractions and a bike ride at a leisurely pace.
Yes, I second Kooridoor Tour, but be sure you choose one that goes to the JSA (the price is always the 90,000W).