Mine werent senior pictures nor engagment, but I did do a few with my cousins. I really like using the outdoors. We used an old shed/barn as a backdrop (so cute!), Trees, a bridge, a park! Endless options in the outdoors! As for indoor props, for senior pictures, (girls) maybe a flower or something girly. And for guys maybe like a football (basketball/soccerball depending on what sport they play).
Musical instruments, if they play them (get them to bring their own). Sporting equipment (footballs, hockey sticks etc) - whatever they're into. Old cameras are also cool, if they're into photos - you can get them to pose as though they're taking your photo, with an old box brownie or something.