martial arts, anatomy, art, dumbek (even if you just learn a few rhythms, this helps)

accounting!!!
acting workshops
workshops on promoting/advertising sponsored by the local chamber of commerce

not in a structured/class format, but mediation skills, public relations, networking are all vital to the business side of dancing.
need to look into grant writing...

Web and graphic design
Quickbooks
Being an office manager for many years helped with organization and communication

And then theater and music, casual and professional aspects of each.

Adobe Photoshop! Accounting...marketing...

Hi!
What a great question! I've found these things to be the most helpful to me. :)
Other dance styles, particularly ballet and flamenco - they really lend an air of elegance, grace and poise.
Acting/expression
Drumming and middle eastern rhythm
Beading and sewing skills

Hope this helps!

I've been an accountant since the 1980s so that helps immensely.

I also got my national certification in massage therapy which helps a LOT with the anatomy and breaking things down for my students. Plus the training on that is ongoing, I have to keep up my continuing education to keep up my license.

Other dance classes - Rhuma, Flamenco, Salsa & Mexican Folk dance (really BIG skirts).

I see you wrote "outside of dance classes & workshops"... I'd have to say MUSIC lessons help you in your dance BUSINESS, since you will know what you are dancing to (or teaching) better.

Outside of the arts, my typing course has been most beneficial ;-)

Pilates, Musical composition and history (and any other musical classes you can take), Cultural Anthropology, Kinesiology, Athletic training techniques, Education and learning modules, music editing, art classes (endless sources of inspiration and ideas for creative processing), and biz.

Oh and one other, computer programming which was my other major in college. I've been able to build and maintain my own website, although I probably need to update the "look" now. It was cutting edge when I first built it!
:)

Sewing and costume making.
Yoga.
Any type of dance/movement.
Web design (if you can do your own website, you cut a major cost)
Photography (helps with what makeup looks good and what colors look good. Plus, all the time I spend looking at photos of other dancers helps me improve my presentation)

For marketing, web design and graphic design. There are way too many fantastic dancers with bad sites, bad business cards and no "brand" to speak of!

For teaching, my fitness certification has been invaluable, as has my yoga practice, and my training in ballet, jazz, modern and ballroom. Different disciplines give you a different frame of reference for teaching and creating dance-- and coming from such a structured dance background has helped me give structure to my classes.

Anything that relates to business and marketing. A lot of community centers have entrepreneurship courses, which can be invaluable. Web design and graphic design can be useful. Classes on photoshop are great. Yoga was a great help while I was taking it, but I haven't been able to find a class that works for me since I moved. Some community centers have classes on tax-prep for business owners, and knowing how to file your taxes properly is always useful, as is anything on accounting and book keeping.

Strangely I've also found that studying meditation and reiki have helped greatly. I've learned some great relaxation techniques that have both helped me keep calm before a show and have helped my students relax their bodies so they can dance more freely.

... Second the above who mentioned computer skills. More and more chores are automated these days, and you will be expected to do them yourself. Web site, photography, photography to posters and publicity, music cd chopping and fading, mailing lists, internet searches, email management, word processing, data backups ,hardware selection and purchase, financial and tax packages. I am the dance amateur in our 3-person business but my computer skills have been an absolutely essential complement to the teaching and performing.

And the fun part about it, is that every thing changes every 18 months! If you keep yourself comfortable with computers you'll bless your skills again and again.