Cross training. Some of the best athletes use it. Football players in a ballet class has to be quite a sight! Flexibility and coordination may be gained from ballet class. Ballet can also train a person to have more kinesthetic awareness or even aesthetic awareness, or the lack of it. Some athletes engage in related or unrelated sports: golf, baseball, track, tennis, swimming, etc. All can enhance aspects of strength and coordination which can add up to control and endurance being improved.
What are we getting at? How can cross training apply to music and to writing music? I've met many musicians who play more than one instrument. Most have a favorite instrument and it will usually be the one that is their best. Still, there is much to be gained from learning other instruments, even if it is more like a hobby in the approach. In college, classes for music majors can include: percussion techniques, woodwind techniques, string techniques, brass techniques, and vocal techniques. In those classes you can learn a little about the instruments and gain some familiarity with how it feels and sounds to play them.
Cross training can apply to finding inspiration. Anything and anyone can be inspirational. If not participating in other arts, I can still be inspired by them. Maybe I'll take more dance classes. For now, maybe I can watch dance on YouTube or go to a live performance. Museums, galleries, and exhibitions of paintings, drawings, and sculpture will inspire me. A recent one I attended is gigantic. TEFAF was in Maastricht. You enter and see some shops with gorgeous paintings and sculpture and speak with people inside. Awesome! You are not even in the main area yet. Thousands of square feet of exhibits by names you should recognize, many you will not, and the art is from all over Europe. It is too much to explain. Several restaurants and bars make it possible to enjoy the day even more, not having to leave for a drink or a bite. It is also a social event and the interaction and reactions of and with others just add to the experience.
If you're not inspired by art or other arts, travel may be great cross training. Inspiration doesn't always show up at your doorstep. Sometimes you must step away and seek inspiration. Every continent has great places to explore and discover. I've only seen 2.5 so far and a few islands in the Caribbean. For me, Paris, Venice, Rome, Tuscany, Munich, Aachen, Amsterdam, Maastricht, and Barcelona were some high points for experiencing culture, architecture, art, music, and food. Food can be inspirational.