While Lucerne is an incredible city, under the circumstances I'd suggest skipping it this time. Given the fact that you won't arrive until late afternoon, by the time you get to Lucerne and checked into a hotel it's going to be about supper time so you won't have a lot of time to see anything.
While Zürich doesn't have the same character as other places in Switzerland, there are still some sights worth seeing. You might consider staying at the Walhalla Hotel, which is right beside the main rail station. Many of the city tour buses depart from the lot right across from the hotel, so you could book a short tour of the highlights or perhaps a short cruise on the lake. If you don't want to tour, just do a walkabout in the neighborhood or perhaps some window shopping. There's a huge Rail City mall under the station, and some nice stores nearby (including an Apple store, if that's of interest).
Following that have a nice hot meal and by that time you'll probably be ready for a good sleep. In the morning, walk back to the station and get a train to the airport for your flight to Greece. The trip from the Hbf to the airport is only about 10 minutes. It doesn't get any easier than that!