I don't know your taste, so just list up couple of suggestions.
If you'd explore outside the airport, I do suggest you buy some good guidebook beforehand.
3-hour shopping tour to Rinku-town mall by bus
5-hour tour to Kishiwada castle by train
http://www.kansai-airport.or.jp/news...age/3htour.pdf
Osaka downtown is just 45 min train ride from KIX, so you have enough time to visit virtually any corner of the city, including Osaka castle,
http://www.tourism.city.osaka.jp/en/...cas/index.html
Floating garden at Shin Umeda Sky-building,
http://www.skybldg.co.jp/garden/e/index.html
Dotonbori, Tsutenkaku tower, etc.
http://www.tourism.city.osaka.jp/en/
In the airport, you can day-use Hotel Nikko KIX with swimming pool and gym
http://www.nikkokix.com/e/top.html
or kill hours at Observation Hall, just watching runway.
http://www.kansai-airport.or.jp/en/s...index.html#_01