This is an interesting activity that you can do with your 2nd, 3rd and 4th grade students in about one class period that will deeply impact their understanding of time. You see, we spend a lot of time teaching children how to read time, how to add time, how to subtract time, yadda, yadda, yadda, that we don’t think about teaching them some of the properties of time. For example, that as Albert Einstein showed, time is relative. Of course, teaching general and special relativity to young children is a little bit above anybody’s pay grade, but having kids write about the relative nature of time can only help them understand it better.
This activity is designed to help students develop an understanding that time is not an "absolute." That is, a minute is something that is neither a long amount of time or a short amount of time, but merely a unit of time. So if you have a minute and have to zest a lemon, well, a minute is a long time. But it you have a minute, and have to prepare an skillet of caramelized onions, well, that's just not enough time. Incidentally, I doubt any of your students will come up with these examples.
My students really enjoyed doing this activity, because it gets to their intuitive understanding of time: that our perception of time is entirely dependent on the activity that is taking place. They have fun brainstorming the different activities that "feel" like a long time (an hour in traffic, a minute waiting for a friend to arrive for a playdate, etc.) and activities that feel like a short amount of time, like a minute playing a video game or an hour for a play date with a friend (which I'm sure the parents feel like goes a lot slower.....)
I've included a sample for you to look at. This is a great activity, you can do it in about 30 minutes, and it will get your kids writing about math!