What airports have you been to where you can watch traffic coming from or going to other airports (small, private airfields don't count)?
A few examples that come to mind:
At ONT (Ontario, CA), the westbound approach to LAX flies right overhead. You can watch the planes going to LAX (albeit at high altitude).
At SNA (Orange County, CA) you can look to the north, and see the same flight path going to LAX if it's a clear day. Also, at SNA, the planes (what few there are) going to LGB (Long Beach, CA) fly right overhead. You can follow them in the sky as they make their final run downwind. The planes usually fly low enough to make out the type and airline.
At JFK, you can frequently get good glimpses of most approaches going to LGA, just 8 miles or so away to the north.
At LGA, you can look to the west and see the planes going to or from EWR (depending on which way traffic is flowing)
At SJC (San Jose, CA), on a clear day, you can look to the north, and watch many planes funnel in towards SFO.