Where in Arizona? I-40 takes you to northern Arizona, I-30/I-20/I-10 funnel into southern Arizona. From Pennsylvania you have lots of options to get to either. There are thousands of possible routes, and the best route will be the one that goes through the places you want to stop.
You can stay as far north as I-80 and work your way down through Wyoming, Colorado and New Mexico to either I-40 or I-10.
You can get on I-70 into St Louis to pick up the Route 66 trip to northern Arizona. You can go down to I-40 on one of the Interstates in the East, see Nashville, Memphis and Little Rock. From Little Rock you can take I-40 west to pick up the Route 66 trip at Oklahoma City, into northern Arizona. Or you can go southwest through Hot Springs, Dallas, Odessa to join I-10 in West Texas for a southern Arizona destination.
Getting off the Interstate system, I've used US-24 going into northern Colorado (intersects I-80 around Toledo), US-36 across Missouri and Kansas into central Colorado, or US-60 across Kentucky, Missouri, Oklahoma, Texas, New Mexico and into Phoenix (I've actually taken that one all the way from Virgina to New Mexico). US-64 and US-54 will get you from the Midwest to the Southwest on diagonals. US-50 will take you through Kansas City, across southern Kansas and southern Colorado.
It all depends on what you want to see, you'll find routes going through just about anywhere between SW Pennsylvania and southern Arizona without much more than 300-400 miles difference between longest and shortest routes. Do you want to see Chicago, go through Indianapolis for the Speedway, Nashville for the Opry, St Louis for the Arch, Memphis for the music history? All are possibilities. If going to southern Arizona, New Orleans and San Antonio can be on the route. Or maybe Rocky Mountain NP, or Denver, or Colorado Springs and Pike's Peak, or across southern Colorado to the canyonlands around Grand Junction and Moab, and down through Monument Valley to the Grand Canyon.
You'll have to choose a few key sites and route through them, because three weeks is not enough time to stop and see everything interesting on any route that gets you there and back. It took me three weeks from Guthrie across US-50 into the canyonlands, down to Monument Valley and back through NW New Mexico and the Texas Panhandle. I would have needed another 3-4 days to add the Grand Canyon to that trip.
I suggest you put a rectangle with one corner where you start, the other your Arizona destination, expand it a little bit, and start collecting tourist information from all the states inside the box, because they would all be potentially on your route. If you are thinking about Mount Rushmore or the Black Hills, expand the box north to cover South Dakota (which is somewhat north of Pennsylvania) and you'll find Wyoming on your trip possibilities too.
Put your list of major interesting places on a map, get hooked up to some trip planning software, try connecting the dots, maybe in a loop. Let the software figure out the trip time, and start prioritizing your stops, so you can eliminate places until your trip gets pared down to the time budget. Then let the planning software search for the interesting places along or near the route that connects the important places, start over prioritizing and weeding those. A lot of times though, minor interesting places make good overnight stops.
For the past ten years (my return to U.S. and retirement) I've used Microsoft Streets and Trips for this task, because it worked well, was easy to use, had a good "interesting places" database and could search up to 30 miles parallel to my route, could budget time for all stops, and quickly recalculate after any change. Microsoft has decided that S&T is no longer commercially viable, so you'll have to find something else.
I'm not going to recommend any places to stop. Your interests may be nothing like my interests. I find that I can spend weeks, sometime years, visiting and studying places that people with other interests simply can't drive across fast enough to not waste anymore time there. The grasslands of the Great Plains comes immediately to mind, and a lot of people also can't find anything to appreciate in the southwestern desert.
Natural wonders vs theme parks, museums and libraries vs roller coasters and giant water slides, pre-Columbian ruins vs Civil War battlefields, small midwestern farming communities vs the largest shopping mall in the U.S. We all have different tastes. That's why you collect information about where you might go through, decide what interests you, so you can decide what you will go through.
Tom Test
Itasca Spirit 29B
