Which would be the best sightseeings in Chicago?

The Hancock Building has a magnificent view; Ed DeBevic's is a fun restaurant. The Museum Of Science and Industry is very fun. I enjoyed the boat architectural tour, but your dad might not be able to handle it.

What does he like to do?

Museums would have wheel chairs if he can not walk for a long distant.

Tour boats or buses are a great way of getting around without walking.

Take him to Sox or Cubs game

Unless he can walk well, it will be a bit tricky for him to get around.

Generally, most tourists like to see the museums, downtown parks, Michigan Avenue shops, the Lincoln Park Zoo, and looking at the architecture. There are tourist buses and boats that can accommodate him.

I recommend that you get a travel book on Chicago, and try to pick the places he might like to visit.