What kind of traveler are you? Seoul provides some spots for different types of travelers. If you're like me, then you might want to go to their National Museum or probably Gyeongbokgung Palace. Seoul Tower might also be a good place to visit. If you're on the younger side, you might also want to consider experiencing the night life in Hongdae or Itaewon. If you're into amusement park, then there's always the Lotte World.
According to my experience, the weather will be hot. The thing is since it's not really humid in Korea, you might feel that it's hotter compared to Malaysia. Shorts and cool t-shirts will probably be your best friends while traveling during that time.