Here they are after the break:
This one may seem a little obvious, but you'd be surprised at how often it gets overlooked. Introduce your child to fun music books from their favorite movies and artists, such as Harry Potter, Star Wars, Lord of the Rings, The Beatles and many more. Even if they are playing in school and/or taking private lessons, sometimes that music isn't enough to get them motivated. The music they get in school will probably be either band/orchestra method books containing mainly folk songs and short ditties the author came up with or parts intended for group playing which don't tend to be a ton of fun to play by yourself at home. On the other hand, the music they get in private lessons can be technical and academic, which can also be difficult to stay focused on. The fun books provide the child with a break from their other music where they can just enjoy playing some of their favorite tunes. It can also be used as a treat to allow the child to pick out a new book as a reward.
Have your child listen to a wide variety of recordings featuring their instrument, from classical to pop/rock. This could mean taking your child to the music store to pick out some CDs, have them sample music on iTunes, let them download some music they like, and watch videos with them on YouTube. Here are some great YouTube videos of popular music being played on classical instruments your child might enjoy.
Take your child to see a symphony or band in your area. Summer pops concerts are great because the concerts tend to be outdoors, feature popular music, and are usually inexpensive or free. The LA Phil at the Hollywood Bowl has its annual Looney Tunes and John Williams concerts and collaborates with various popular musicians.
My next suggestion may also seem a bit obvious, but I thought it would be worthwhile to mention; private lessons. Working with a private instructor will take your child out of the group environment at school and allow them to work one on one with an industry professional. School bands and orchestras are great places to start, but your child might be hindered by peer pressure, loud distractions, and a learning pace that is either too slow or too fast. In private lessons the student can set their own pace and feel free to ask any questions they want without fear of of what their classmates might think.
Youth band and orchestra programs provide an environment for more serious and motivated students to interact. They supply harder music and coaches to help and challenge students. The orchestra coaches are usually private teachers themselves and frequently perform in the area. Youth programs also offer chamber groups, which are usually not offered in schools. I also want to include summer camps as well. Besides offering musical enrichment, a lot of them also offer outdoors activities such as swimming, boating, tennis, basketball, etc. I know these programs come at a cost, but most do offer scholarships.
So my final tip, may also be the best because its 100% free. Tell your child to invite over a friend or two who also play instruments so they can play together. Don't try to over control the situation by rehearsing them or giving advice, even if you have some musical experience yourself. The point of this activity is just to let them play music with their friends in an unstructured environment. In school and in lessons they are always being told what to play and how to play it. This way they can just experiment and have fun playing their instruments with others.