Wow, long post warning . . .
Ok.
I'm surprised no one's mentioned Mixology. It looks daunting, but it's not really that hard with a bit of effort.
I'm doing that in my Dynasty. You can't start until teen, but once you do, the only really hard challenges there are serving 60 sims (so drag the family with you to the bars and get them to order) and the 42 custom drinks. (Take the observant reward and you'll usually learn them 1rst or 2nd try.)
See, once they're a teen, have them make drinks until they can make skill gain drinks. Then drink those to boost their learning time. Once they can make new custom drinks, get them to make those -- they'll still be boosting their Mixology skill, and not leaving drink platters everywhere! Once you finish that challenge, go back to making mood drinks. The skill gain drinks will help out anyone, so they're actually really useful. The energy drinks too, since they've basically replaced coffee for my Dynasty.
After that, just have them make as many mood drinks as possible until they're a Young Adult. Then send them to different bars to moonlight and learn custom drinks. The 200 mood drink challenge will probably already be finished, and they can learn drink recipes while moonlighting. If you've got a shortage of patrons, just send the family or friends with them.
My Mixologist is a few days away from Adult, and the only challenges I've got left are; the 42 custom drinks one, which I only need one or two more drinks for, and the 500 drinks one, which considering I'm at 400-something, shouldn't be too hard.
The only real problem I found with Supermaxing Mixology is it doesn't tie in easily to a career. My Mixologist is also a stylist, I can easily also see something like Mixologist/Buisness. Also, (and this may just be me, not sure) I'm not getting Mixology Opportunities in Sunset Valley, so I'm having to use her backup skill (painting) for the opportunities.
- - -
First of all, how easy is martial arts to supermax?
Relatively Easy.
Just make sure you can still travel, since you -have- to do the tournament in China. I was going to have someone Supermax Martial Arts, but since I can't travel in my dynasty any more, that's out.
Though Athletics is a nice backup.
The other thing to consider with martial arts is that Sims build athletic skill while doing martial arts, so it may mean that the athletic skill cannot be a supermax skill for another immortal.
The only exception to this being Meditation, which does not raise the Athletics skill.
I don't know if it's possible to complete the Martial Arts skill challenges (I'm specifically thinking of the one to break 150 boards), without maxing Athletics. The last sims I did that with had the Physical Perfection Lifetime want, so I didn't care if they maxed athletics too. I'd be worried that you can't do the sparring and board-breaking challenges without getting Lvl 10 athletics. Hmm. . . something to test, I suppose?