4th grade we do "Recorder Karate". This is the greatest elementary music innovation EVER in my opinion (a small school band director that reluctantly teaches a few elementary classes). Look it up - kids love it.
We start band instruments in 5th grade, which is a little young in my opinion, but does have its advantages - kids are still really excited about playing "a real instrument" after a year of recorders. We do a very limited set of instruments (flute, clarinet, cornet, trombone) then branch out from there in 6th grade (percussion, sax, baritone, tuba, F. Horn)
When I've had younger groups, we've gotten a lot of mileage out of fun songs on YouTube and patriotic music as well.
Hope this gives you some ideas - good luck!