High above the old town, the 35-meter-tall Diana Lookout Tower (Dianaturm) was built in 1914 and provides fantastic views from Friendship Hill over Karlovy Vary and its surroundings (on a clear day the views extend up to 70 kilometers).
If you're up to it, take the well-marked trails from the town center to the base of the tower, a 30-minute walk that's well worth it, as you'll pass numerous historical buildings.