One interval that students must focus on getting in tune is half steps. When F# to G is well in tune, with a high enough F#, it makes all the difference. The half step interval also helps when students are learning E to F natural.
I teach the half step interval during the first couple weeks of school by having the students play JAWS. We make it quite dramatic and students must only play when I am moving my bow...that way, we can stay together. After we get faster and faster, I stop playing and plug my ears and the entire class screams as if we are being attacked by the monster shark, JAWS.
One thing to make this really fun is to buy one of these remote control flying sharks:
For a limited time they are pretty cheap on this website: (I just bought 3 to auction off in my class)
These sharks are hilarious and so much fun. They are basically a balloon that you fill with helium at then attach a remote control device to make it fly. I make students earn the appearance of the shark by practicing a certain amount of time or performing well. We fly this shark down the hallway while playing JAWS in the classroom. Students love it...and they will remember half steps forever.