I also teach middle school, including 3rd grade.
Do you have mini whiteboards available? There are so many guessing games you can play with those where they work in teams of 4. I find they really get engaged. You can easily adapt it to suit the target language. For example, if the key expression is "In my opinion," have them guess weird facts or geography trivia or cultural trivia, anything like that (multiple choice). Each team writes their answer on the whiteboard, then all teams hold up the whiteboard and say out loud, "In my opinion, ~~~."
The best part about this type of game is it can be shortened or extended so easily (just make a ton of questions, but you can stop playing at any time!). Sometimes I throw in "bonus" rounds or different types of questions (e.g., unrelated to the key expression/today's topic) to mix it up and keep the kids from getting bored.
If you don't have access to mini whiteboards, you could improvise with clear page protectors or laminated white paper and dry erase markers.
I've also done a 'listening' challenge where they have to draw a picture according to my directions, then I get them in groups of 4 and give one person in each group a new set of drawing instructions, which they have to read to their group slowly and clearly while the other 3 students draw. (You can use that just for fun or to practice "Would you say that again? / Would you repeat that? / Pardon me?" type of expressions.) Takes 10-15 minutes depending on how complicated you make the instructions.
Bingo/"find someone who" mingling activities usually take about 10 minutes (maybe a little more if you have a big class).
Hope this helps!
