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 most tourists do not know 🙂 You can combine a visit to the bridge perfectly with a stroll or run along the Main river. Take a break or end up at the lawns in front of the Hafenpark (check the Hafenpark article), or have a coffee or snack at the “Oosten” restaurant, which is nearby and a nice location, especially on sunny days (might be VERY crowded with locals, though)
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 (this would be “Eisener Steg” with its love padlocks, which is also a lot more crowded) due to the frequently crossing trains, but you’ll be rewarded by the view!