Cityscapes

Where would be the best location for cityscape photography in Tokyo? I would like to have some open space in front of me after which the city 'erupts' in the distance.

maybe somewhere on the Tokyo Bay or Boso Peninsula??

Go to the Roppongi Hills at evening towards Asahi television station. On a balcony you can easily see Tokyo tower along with other buildings. Also, heading towards Yokohama (if you aren't strictly Tokyo) you can take great photos of Landmark tower and the Cosmo World ferris wheel (also a spectacle at night) which is right next to the tower.
If it's ok to stay on the ground and you don't need to be high in the air, try going to Shibuya (night or day) and capture the busy crosswalk and the large and bright television screens on the buildings.

how about the Tokyo Metropolitan Government building, located on the western side (nishi-shinjuku) of JR Shinjuku Station. you can't miss it with its twin towers raisng from a huge base . there is a free observation desk in each of the 2 towers. check: www.metro.tokyo.jp/ENGLISH/TMG/observat.htm - .

Odaiba you can get a nice view of the rainbow bridge, with tokyo tower and various buildings.

A scene I kind of like is the grounds between the Imperial Palace and the Marunouchi area. It's like a huge park with nothing but grass and pine trees. If you get on the palace side of the lawn and face the tall buildings of Marunouchi, it's kind of dramatic, I think. Not a lot of variety, but that's sort of why I like it.

Shinjuku Gyoen has some interesting places where glimpses of skyscrapers contrast with a nature theme.

I have a couple of days in Tokyo and I want to try some late afternoon cityscapes and night time bokeh shots.

everywhere in Tokyo can be an excellent location for cityscape photography and most tall buildings have observation decks for a bird's eye view of Tokyo.