I’m a big fan of Stadel’s Black Box. I think what makes it fun is that there’s something small to figure out (What does the black box do?) before figuring out the big thing (What’s the sum of those two fractions?)
I recently did this with my fourth graders, and it was a ton of fun. Here were some of their answers to 1/2 + 1/3:
3 1/2 / 4 (three and a half fourths)
Can you figure out how kids got each of these answers?