Family outing spots perfect for visiting in the fall

Since my in-laws live in Busan, I often go down there, and my husband is quite the filial son, so it feels like we are visiting places we've been to many times.

The places I go to often are Eulsukdo Park and Dadaepo Beach. If there are other good places for family outings besides these, please recommend them.

Preferably, a place with many flat areas.