One of the best places to see Vienna is from Kahlenberg, which is 484 meters high and lies in the northeastern foothills of the Eastern Alps. You can see the breathtaking view from the nearby restaurant. There are also souvenir shops, restaurant and Kahlenbergkirche (Kahlenberg Church). This is one of the places worth visiting for the views so don’t miss it!
Kahlenberg is a small hill in the North of Vienna (19th district) with a panorama platform that gives you a very nice view over the city. If you go there in autumn/winter I'd recommend wearing warm clothes there, as it can get very windy and cold up there.
If you go on public transport, you'd take the bus 38A from Heiligenstadt, which is the last station of the U4 (underground) in the north.
If you go by car, you'd drive out of Vienna through the Handelskai. Continue until you reach Klosterneuburg und then follow the road signs towards "Höhenstrasse" and "Kahlenberg". The other way to go would be through Döbling and Grinzing, but I usually prefer the first option.
You can get a great view of the city from Kahlenberg and (even better) Leopoldsberg, two green hills in the Western part of town. The hills are also popular with the locals for Sunday walks in the forest.
On the way there, you'll pass through one of Vienna's major Heurigen areas. It is quite touristy but still OK if you avoid the places where all the tourist buses unload - generally, the smaller and simpler a Heurigen looks (sometimes they just look like regular residential houses with a few tables in the garden), the more likely it is the genuine thing and not a tourist trap.
Another interesting thing on the way up Höhenstrasse is the Krapfenwaldbad, one of Vienna's nicest public outdoor pools (it's a bit more expensive than others, though).
If you have some time, I suggest to take the bus all the way up and then walk down through the woods (takes about 1 hour, trails are well-marked).
Take the 38 Tram at the Schottentor (Universitaet) and go to Grinzing. Then take the 38A bus to Kahlenberg. You will be surprised to see the whole city from above, all the bridges, Donua and even till Hungary.
While your at Kahlenberg, Do board the Hueringen Express! The train goes through Nussdorf, Kahlenberg & Grinzing.