Wow, we are giving a lot of confusing advice.
In my experience, the minor airports (Trenton, Allentown, even Hartford and Albany) are inconvenient in terms of times, or more expensive, or both (Hartford and Albany). Coming from California, I would stick with Newark, Philadelphia, or BWI if possible as the target airports.
I agree that seeing too many campuses in a short period of time produces overload, but around Philadelphia especially the campuses are really not far apart. If you wanted, and didn’t care about taking the scheduled tour, it would be easy to visit three in a day, and even maybe to do a drive-by on a fourth.
Some clusters, with a Philadelphia-centric orientation:
Penn and Drexel, as everyone has said, are separated by a four-lane street. Travel time between them is about 20 seconds, plus another 10-40 seconds waiting for the light to change. They are a 15-20 minute drive north from the Philadelphia airport, with normal mid-day traffic. Swarthmore is 15 minutes mainly west from the airport. Haverford and Villanova are five minutes apart from each other and about 20 minutes from either Swarthmore or Penn. The University of Delaware, which someone mentioned, is maybe 25-30 minutes driving southwest from the Philadelphia airport or Swarthmore.
Princeton is about an hour northeast of the Philadelphia airport, 45 minutes or so from Penn. Lehigh is a little more than an hour north, and Lafayette 10-15 minutes east of Lehigh. Princeton and Lehigh/Lafayette are probably about 30 minutes apart. About half way between Philadelphia and Lehigh, a little off the most direct route, is Ursinus College, which is probably worth checking out if Franklin & Marshall is a prime target.
Franklin & Marshall is about an hour west of the Philadelphia airport. If you were a crow, you might visit UDel and F&M, but the roads don’t work so well for that, unless you really love driving on two-lane roads through horse country and Amish farms. Dickinson is another hour or so west of F&M.
Towson is over two hours from central Philadelphia, but probably about 2 hours from the Philly airport. A little over an hour from UDel or F&M, maybe 90 minutes from Dickinson. Central Baltimore and Hopkins are about the same distance from Philly or UDel, but 20 minutes more if you are coming from F&M or Dickinson. BWI, the Baltimore airport, is another 20-30 minutes south of there. The University of Richmond is about two hours south of there.
Syracuse is four hours north of Philadelphia, maybe a little more depending on where you start. If you are starting at Lehigh, take an hour off that. Cornell, which is physically much closer, takes about the same amount of time to get to, maybe even longer, because of the dinky roads in and out of Ithaca. If you drove to Cornell from Dickinson or Franklin & Marshall, you would take a different route, it would be about a 3-1/2 hour drive, maybe less, and you would drive right by Bucknell on the way. Ithaca College, 15 minutes from Cornell, may also be worth checking out. Cornell to Syracuse will take about 1-1/2 hours. If you swung slightly out of your way, one Finger Lake west, going from Cornell to Syracuse, you could visit tiny Hobart and William Smith College (that’s one college, not two . . . anymore).
Hamilton and Colgate are each somewhat less than an hour east or southeast of Syracuse. Skidmore is another 1-1/2 hours east of there, as is Rensselaer Polytechnic Institute (they are 20-30 minutes apart). Driving directly from Philadelphia, RPI is about 4-1/2 hours, and on the way you could stop (if you wanted, and if you didn’t mind adding some additional driving time) and see Vassar and/or Bard.
Williams (in which you ought to be interested if you are interested in Amherst) is about an hour east of Skidmore. Amherst is another 1-1/2 hours east of there, over mountain roads. Driving directly from (or to) Philadelphia, Amherst is a bit less than 5 hours (but traffic in and around New York City can add to that), and your route would take you past Princeton, Yale, Wesleyan (not to mention Columbia, NYU, Rutgers, Fordham, Cooper Union, Sarah Lawrence, Fairfield, and everywhere in North Jersey). Smith and Mount Holyoke are minutes from Amherst. They may hold a lot of interest for a boy, but not as potential colleges. Hampshire, and of course UMass Amherst, are also close by. As others have noted, Dartmouth is another two hours north of Amherst.