Hey, Mayrhofen is pretty sick although the park can get pretty busy with english tourists who think they own the place. The park is pretty big and if the snow is good they build some good lines. There are some pretty nice side country tree lines if you go to the top of the Harikiri (the steepest piste in Austria or something) and head left from the chair, or head left and ski on piste for about 500m, hang a left again and there are some nice lines when it snows hard.
If you're in the zillertal area I'd go to Hintertux on pow days, I'd say it has the best back and side country for when the snow is deep. They don't groom everything and although you're still on piste it's like being out back. We found some great lines there last year. Be careful though, something crazy like 600 people die a year in austria as the bc is pretty uncontrolled, have a look at the ava risk before you head out there. The park is pretty sick in Tux as well and has a much more mellow vibe than Mayrhofen (penken park).
Hochfuegen is another place you should try when it snows.
Have fun out there, there's some sick terrain in Zillertal.