Students stay in their seats, facing the teacher, unable to view others' responses. The teacher gives a very brief lesson on run-ons v. fragments v. complete sentence. Take time to stress this: That a fragment can be l-o-n-g-e-r than a complete sentence.
Next, hold up sentence strips containing run-ons, fragments and complete sentences. (I incorporated sentences from our current social studies unit. Why not?)
Have students shuffle through their cards and hold up a C, R-O, or F. You begin to get instant feedback on who's "gettin' it" and who's not. I did this with 4th graders, so have not taught diagramming sentences. Instead, I think they can begin to get the "feel" of a run-on/fragment after some practice. I did, probably, 50 sentence strips, and it was quite fun to see almost 100% of them start to get it after about the 15th-20th strip.
Next, I had the boys line up on one side of the room and the girls on the other (spelling bee style). I showed a strip, the next person in line cupped his/her hand around the card and showed me. If they were wrong, that strip stayed "active" and was shown to the next person.