At the end I had a great weekend in Ticino, thank you for all your suggestions!!!
We went for the Wilhelm Tell Express route, which is a great trip by boat from Luzern to Flüelen (thank you Nick) and then continued on a panoramic wagon to Locarno.
We stayed at the garni Muralto, nice hotel, great location walking distance to train station and lake vue, and reasonable prices.
Then we went to Lugano and stayed at the Continental hotel, also perfect location, only few steps from the train station, and great view over the lake. They also have a nice swimming pool in a park.
Weather was excellent, so we had a perfect Ticino weekend