Will be in Portugal in early May for 10 nights. Flying into Lisbon. We want to go to Tomar and then Porto. Is it best to take a train from Lisbon to Tomar to Porto? Rent a car? A combination of both? Not sure what are plans are yet after Porto, where we plan to spend at least 2, possibly three nights. Is that a good amount? I should say that the first 3 nights are in Lisbon and most likely the last night too, which leaves 6 nights. I assume Tomar is good for a morning/afternoon but not overnight? Feel free to convince me otherwise.

Tomar is the most interesting spot in Portugal from the architectonic point of view. Rent a car since Tomar is outside the main railway system and it's not pratical to go there by train. Christ Convent and templar's castle are pretty close to the city (2 miles or so) but a very steep climb and you need a car to reach the place. I'd stay overnight and visit the museum of Jewish heritage (in the city). A few miles away there is Almourol Castle another. Almourol and Tomar castles were the front line in medieval times between christianity (in the north) and the muslim world (in the south).
Until 27th June Christ Convent includes a very interesting temporary exhibition of medieval religious sculpture in stone included in the 6 euros entrance fee.
The last but not the least I attended last month the Restaurant Taverna Antiqua in Tomar's main square. It offers a medieval experience (both atmosphere and food). These kind of places tend to be kitsch but I found an amusing experience and worthwhile the 40 euros for 2.

you are welcome