Your thought is quite polite, that is returning the invitation. Under normal circumstances you inviting them out to a restaurant is entirely valid, but mostly as a gesture of friendship, not directly as 'reciprocating' on Thanksgiving.
Why? Well, Thanksgiving is 'special' in a way that Christmas is not. Partly this is because Christmas might actually be Hanukkah, or some other celebration, and all in all it just isn't the short time period, highly specific holiday that is Thanksgiving. In a normal year, most students would head home if they could. And then, if your roommate or friend can't make it back home (cross country, cross borders), it is totally normal to invite them to your home (or to the parents or grandparents) to be part of the celebration. There is always enough food to go around, and an extra person or two is easily taken in stride.
As an example, right now I have my two children back home from university, and one of their friends who was not going to fly cross-country right now and has too much work due next week to do the drive (after the semester ends they will have time for the drive). We had planned for two additional people as well, but their plans had to change (again, end of semester assignments so it just didn't work out).
But, as the semester winds down, and people have a month or more until classes start back up, more and more people go home from univeristy or go off to visit relatives. The Christmas holiday is just more diffuse.
So, appreciate that you have good friends who would invite you for a lovely Thanksgiving dinner. Treat them at some later point, but not specifically in response to this invitation - just enjoy being able to treat them as friends. If that is possible over the holidays, if you are both in town, even better. But inviting 'extras' for Thanksgiving is pretty well ingrained in the US.