Good advice about having a full tank of gas, but Allentown isn't that far. The CT ferries are a nice ride for sure buy waaay out of your way.
#1 thing to watch out for on Long Island are PARKWAYS. DO NOT take any route that includes a parkway, there are signs that will say NO TRUCKS. Be careful because parkways have LOW bridges, some as low as 8' :E Some exits are very close together, and about once a week the State Police have to tow a semi off of one of the parkways...
That said, we go to PA quite a bit and generally the best route from Allentown is I-78 East to Newark, then I-95 north (NJ Turnpike) to X18 I-80E, George Washington Bridge (GWB). I-80 will split, stay to the left in the Express lanes (unless ETA sign shows big delay then go local side). I-80E will end and turn into I-95N, this is only a marking change, you're on the same road.
Continue over GWB on I-95N Cross Bronx Expressway and watch for signs I-295 Throgs Neck Bridge (DO NOT take the previous exit I-678 Whitestone Bridge even though it will say Long Island.) Continue on 295 to I-495 East, Long Island Expressway (LIE), and that will run you right out to Calverton.
This route that I just outlined is the simplest, best marked, and should take on average 4 hours with moderate traffic. There are some alternatives like taking 287 to 80 in NJ, or going South on 95 to the Goethals and Verrazano bridges, but that the southern route puts you on I-278 Brooklyn-Queens Expy (the BQE) which is torture....No matter what you will end up on 495 LIE once on LI, it is the only limited access highway going East, everything else is either a local road or a parkway.
Tappan Zee (now the Gov Mario Cuomo Bridge) is also waaaay out of your way and not a good option unless catastrophic closure on the GWB because traffic going up 287 and back down 95 from the TZB is not any better.
Regarding timing, be aware that the Allentown area is now considered commutable to NYC, so traffic will build on I-78 and I-80 starting at 5:00 am. Either leave Allentown later in the morning, say 9:00 am or wait until later like 6:00 pm when everyone is heading West. By the time you get to Long Island the LIE should be less clogged.
EZ Pass is highly recommended, we just installed open road tolling on the Throgs Neck and other East River crossings, so if you don't have EZ Pass, you'll need to pay online with a penalty like they do in Chicago. There are tolls on all bridges and I-95 and without EZ Pass you will suffer.
Sorry for the long post, lots of info. If you have questions, PM me and I'll give you my cell #. Happy to help. When the kids were little, Eastern PA was a favorite weekend vacation spot, so the rig in the sig pic has made that trip a zillion times. TT before this one was 2' longer, 34' vs current 32'.
