I have a really fun activity I do with kids when I do a group lesson on Instruments of the Orchestra. I recently did this at a local Christian school I teach at once a week for the 4th and 5th graders and the 6th raders and they LOVED it! When I focus on the brass instruments, I ask them to tell me the order of each instrument (trumpet, french horn, trombone, tuba) from highest pitch to lowest pitch. I usually get a variety of responses so I tell them we are going to "figure it out together" with a fun activity. By using tape (masking or painters tape) or string, we measure out each instrument as if it were uncoiled. Before we do this, I ask them to look at the instrument and tell me how long they think each one is. I usually write down their answers, tell them the correct answer and then we measure it out together (also a good math lesson as we use a yard stick and have to figure out how many yards sticks we need to measure based on the number of total feet!).
Once they see the instruments measured out next to each other, we have a great discussion about how the length helps us determine the high or low of the pitch - which instrument is highest to which one is lowest. This translates nicely into a discussion of the strings on the piano and their pitch - why some strings are longer and thicker than others.
It really is a fun exercise and the kids stay very engaged as we work on it! It's also helps them remember it because it touches all the learning styles - audible, visual and kinesthetic.