AT ORD, there are several great places to spot and take pictures. I went there in October, and saw so many planes it's not even funny! Are you flying or driving? If you drive park in the Garage by the Hilton, if you are flying got to the Hilton and go outside, cross the terminal drop-off area, and go to the top level of the garage; it rains a lot in the midwest in the spring, so hopefully you get a good day! I'm getting my driver's lisence on 30 May 2000, so I can drive to IND whenever I want (I hope), so hopefully my parents will let me fly to ORD, if not on the 30th some time over the summer.
When I went on October, I took 96 pictures in about 3 hours!