Take the opportunity to make a boat trip around T'aejongdae. I can't remember the price but we had awesome weather and then the price didn't matter at all.
The Tour has some comments in Korean only. But mainly they just explain about the islands in this area.
The boat is kinda crappy and small. If you get boatsick easily you might not wanna do that.....
In walking distance from Haeundae Beach is Dongbaek Park.
It's nice for a short walk and offers a nice view at the beach and all the huge hotels.
If you wanna escape from the city, catch Bus No. 30 and go to T'aejongdae Park. It's a 20 minutes ride and takes you out to the nature with awesome view out to sea.