Keep in mind that a lot of vegetarian main dishes will incorporate either beans or eggs as a protein substitute. Quiche works well for this reason, and you can add what ever veggies they like. (This is assuming they do eat eggs!). And Quiche is so easy that it's easy to make two - one with and one without meat.
I made this stuffed pepper recipe this past summer when we were inundated with a bumper crop of peppers and I was running out of ideas!!
http://allrecipes.com/Recipe/Stuffed...l.aspx?evt19=1
Some other meatless main dishes ...
Eggplan Parmesean
Minestrone
Grilled Veggie Sandwiches