Yes, really! It is possible to dive in the mountains in a lake called Erlaufsee which is a few km to the north-west of Mariazell.
The water is supposedly very clear but, I would imagine, extremely cold. Elevation is 828m so you can test your 'diving at altitude' skills as well as your freshwater diving skills. Maximum depth is 38m. According to some information I found on-line there are lake trout, grayling, pike, perch, rudd and charr as well as some row boats and pedalos! Not exactly wreck diving, is it?
Equipment: A drysuit is probably required along with helmet, gloves etc for cold water diving. There is a dive shop there but I don't know if you can rent gear or not.
Skiing is the number 1 sport in Austria and Mariazell is no exception.
It is a common sight in the town to see skiers traipsing along in their ski boots to and from the ski-lift. Within 10 minutes it is possible to be on the slopes of Bürgeralpe.
When I went up to take a look I was about the only person not skiing!
Equipment: Equipment is available to hire (or buy) if you don't have your own.
Cross country skiing is a popular pursuit in Mariazellerland.
Equipment: Bring your own equipment. There is nowhere to hire equipment out in the country.
This looked to me like curling without brooms or a kind of alpine bowling game. Anyway, the participants were having a lot of fun playing.