to decide on a menu for a get-together but they keep changing it.

I think they change the menu 3 times. I cannot remember which episode it was.

I think it's the episode when Jan gets voted most popular girl or maybe it's an earlier episode when Jan wins an award at school for her term paper. I think it's a Jan episode. I could be wrong. I remember Carol and Mike were just going to grill up some hamburgers at first, then it became Mexican cuisine, then a smorgasbord.

Season 4, Episode 22, You Can’t Win Them All. Cindy on a quiz show.

Recap: https://bradybunchreviewed.wordpress.com/2018/06/01/episode-22-you-cant-winem-all/

It was the episode in which Bobby and Cindy both take a test at school to be picked to appear on a quiz show for kids. Cindy gets picked and Bobby does not. Cindy becomes very full of herself after she gets picked. Meanwhile, Mr. and Mrs. were originally going to broil some steaks on the backyard grill for a few friends. Then, as circumstances caused them to invite more and more people, they changed to a Mexican dinner. Then when there became too many people for a sit-down dinner, they switched to a smorgasbord.

I think it's the episode when Jan gets voted most popular girl or maybe it's an earlier episode when Jan wins an award at school for her term paper. I think it's a Jan episode. I could be wrong. I remember Carol and Mike were just going to grill up some hamburgers at first, then it became Mexican cuisine, then a smorgasbord.
Your hunch and reasoning was close. In that episode it was not the menu that was changing but Carol and Mike's second honeymoon. They had to keep changing reservations. It was the beach, then skiing, the something else. When they finally got a reservation Jan tells them her acceptance speech for Most Popular Girl was changed to that night.