Grand Hotel in Zell am See has a spectacular location and great views of the mountains and the lake, but when we stayed there (booked through a UK package company) we were allocated a room in the annex - which had no lake view, no mountain view and we had the distinct impression that the package tour guests were getting a very different and poorer service than the "full price" guests.
So if you want to stay there, book the flight yourselves (easy enough from UK to Salzburg) and then take the train, bus or hire a car - and book directly with the hotel, making sure that you get a room with a lake view.
For alternative destinations, look at Saalbach Hinterglemm, which has better hiking, zip wires, a high rope park and mountain biking - but which doesn't have a lake (although Zell am See is only 20 mins away by car or bus). Or Achensee, Wolfgangsee, or anywhere within an hour of Salzburg.
Be careful around dates. The summer season this year didn't start until 25th May, which means that the cable cars, tourist attractions etc. were not operating before then, which puts a limit on the available activities. Be sure to check the opening dates for next summer - contact the relevant Tourist Offices - before you book.