Pack a picnic and enjoy a fun family day out at Yuridake Park. There's plenty of space for the kids to run around and get some exercise.
You'll have no end of sightseeing choices when in Saikai. Yuridake Park is a good place to start here, but there are plenty of other worthwhile spots to explore. Nanatsugama Limestone Cave, and Sakito History and Folklore Museum are not far from the outdoor recreational space and will fill in your day nicely.