In the summer I would plan seeing an opera on the water in Bregenz. In Burgenland theres Oper im Steinbruch which is their version of Red Rocks.
Imst in Tirol has the world's 2nd longest alpine coaster / toboggan. But there are a ton of other ones around the Alps region too. Many of the ski slopes do downhill bike rentals for thrill seekers. Places billing themselves as "sporthotel" typically have outdoor packages and facilities.
Stift Engelszell near the Passau border hosts one of the few trappist breweries in the world, and the first outside of Belgium/Netherlands.
Vienna has a stupid amount of free outdoor public festivals/events year round, but especially in the summer: e.g. music festivals, arts performances, wine/food celebrations, red bull sponsored activities, avp volleyball, outdoor soccer tournament projections, film projections (multiple types and events). Check Wurstelstand for the schedule.
Popular local thing to do is hike around the vineyards on the foothills at the edge of the city where you can eat and dine at the taverns (Heuriger), often set outdoors.
In comparison, Bratislava is pretty boring from a tourist perspective, but if you want to go there you can take the hydrofoil on the Danube and that in itself can be an attraction.