I know that Oktoberfest is quite a way away yet. However if you are in München and want to visit Theresienwiese where Oktoberfest is held then it is a quite simple way to get there.
When I stay in München I am a little to the north of town. I take the S-Bahn to Hackerbrücke. I know that you can go to the Hauptbahnhof and take the U-Bahn U4 or U5 and get off quite close to Theresienwiese but from Hackerbrücke you are only about a half a mile away. You'll have to climb the steps up, once you reach the street turn left heading towards the restaurant Il Castagno. Just past the restaurant is a footpath called Kurt Härtel Passage. Follow this until you reach either Theresienhöhe or Martin-Greif Str. These will take you to Bavariaring which is the road around Theresiewiese.
This is a big park during most of the year. During Oktoberfest it is
filled with rides and the beer tents. Otherwise the only structure
you'll find there is the statue of Bavaria. During the summer months it
is possible to climb inside, for a fee.
To the left is a photo of Theresienwiese. Near the right edge just below center you'll see the statue Bavaria. This will give you an idea of the size of how large this park is. If you were to follow more to the right you'd be able to see the Kongresshalle and the Transportation Museum. From this view St. Paul's church would be behind the camera.
If you come from the train station you will see St. Paul's church to your left. Here is a link to the church in German http://www.erzbistum-muenchen.de/Pfarrei/Page010890.aspx. There are some great panoramic views of the church in this link.
If you are interested in transportation then the Deutsches Museum Verkehrszentrum. I spent the better part of a day wandering around this museum. There are lots of lockers to secure your goods. You'll need to remember that you'll get your money back once you open the locker again.
There is a small gift shop if you are interested in picking up souvenirs.
There is also a small snack shop that served pretty good food. Restroom facilities were though out the museum.
There were lots of interesting car, motorcycle and train displays. There are lots of interactive displays for both young and old so be prepared.
You are permitted to take photos there without using a tripod and for private use only. Any other types of photos you will need to contact their Press Office. A link is provided in the link below.
If you are interested in the Flea Market then directly across from the Verkehrsmuseum is a building called Kongresshalle. Some Sundays you will find a Flohmarkt being hosted there usually opening around noon.
I have had the opportunity to spend time here twice. Once during Oktoberfest and once during winter. I enjoyed myself both times. During Oktoberfest I just enjoyed being there. It was the first time I was there as an adult. In the winter I just enjoyed the space and watched people with their kids playing in the snow. Plus I discovered many other side attractions as I mentioned in this piece.
Travel safe travel smart