Not only tourists are fascinated by the skyline of Frankfurt- I love to experience it at any time of the year, especially at dawns, sunsets or at night.
I think the “Deutschherrnbrücke” is the top location, and you can combine this one perfectly with a stroll or a run along the Main river, and pause or end up at the lawns in front of the recently fully opened Hafenpark (check the Hafenpark article), or have a coffee or snack at the “Oosten” restaurant, which is nearby and a great location, especially on sunny days.
Start from any point close to the river, whether it’s the Mainkai after visiting Paulskirche or Römer, or from the other side of the river at Schaumainkai with its museums.
Be sure to choose the right bridge to cross the Main as there are a few: The “Deutschherrnbrücke” is the one which is also crossed by trains. It was first built in 1911, destroyed during the 2nd World War and rebuilt in 1949 (some history, if you don’t mind…)
It may not be the most romantic one (this would be “Eisener Steg” with its love padlocks) due to the frequently crossing trains, but you’ll be rewarded by the view!