I took a day trip to Melk on the Kombiticket--train from Vienna to Melk, toured the abbey and had lunch in the charming town, then by boat on the Danube to Krems where I caught the train back to Vienna. There are variations possible with this, check the OBB website. There are also guided tours but I did this on my own with no trouble.
I also took the train to Salzburg for a day--early train got me there by 10:30, spent the day wandering about happily in the Old Town and touring Mozart's Geburthaus and Hellbrunn (the HoHo bus was good for getting to out-lying areas), had an excellent lunch at Triangel in the Old Town, enjoyed a craft fair that ran along the river and then caught the train back to Vienna around 6:30.
I spent 7 days in Vienna and I'm also a huge museum nerd--you won't lack for great museums to visit, that's for sure! I visited 9 and had to leave the Kunsthistoriche for a return trip.