haha! i just happen to have the map for Zillertal spread out on the floor in front of me.
i passed through Mayrhofen/Zillertal on my way to Pfitscherjoch or Tuxerjoch pretty much every week last summer.
a few of the lifts there accept bikes, penkenbahn being one of them. the buses, too.
there is a bike park of sorts. two purpose built trails, but they don’t compare with the likes of Leogang or Saalbach a few valleys away. supposedly good fun with berms, drops, north shore but relatively short at 300m vertical/2.6km and 650m vertical/4km, respectively.
a colleague had a spare day there and spent it on the Penkenberg. he was smiling when i saw him that evening.
there are so many well kept trails there that you can ride. lots of hiking tourists means lots of trails!
i think you could link up lots of other trails to make for a good few days big-mountain riding.
have a look HERE
it’s all in german, but the main thing you need to know is that singletrail is singletrail, leicht is easy, mittel is middle and schwer is hard. flick down to the end of the blurb on each tour and you can download the route onto GoogleEarth.
you can also download the Mayrhofen Bikeguide, but again, it’s all in German.
try and pick up Kompass maps #34, #37 and poss #28 if you think you might drift a little further eastwards.
i’d be interested to hear what you think about the place.
if you’ve got any questions relating to the language email me (see profile). best of luck!