Of course you should mention your date and interests.

It makes a difference if you'd go next month or in summer. It makes a difference if you'd like to do just train trips or boat trips / bus trips / cable car trips, too (you did write "cruise tours"). It makes a difference if you'd like to stay in that area or if you'd like to do longer trips. It makes a difference if you are holding a railpass or if you plan to buy an other ticket.

Oh thanks for pointing out. We plan to come from April 12th to April 17th

We plan to buy a swiss rail pass.

Travel by Train to a point where we can make boat or cable car trips and return back to Chur. 

You should go to Arosa by train and then up to Weisshorn or Hörnli. But you must visit Arosa on one of the first days as the winter season ends on the 14th of April. 

https://arosalenzerheide.swiss/en/Arosa/Winter/Mountain-Experiences

From Chur you also have the Berninaexpress to Tirano. No need to make a reservation. Either use the reservation free coaches that are less crowded than the coaches requiring reservations on the Berninaexpress train or use the regional train that run on the same route. 

The trip takes about 4,5 hours so you can go back to Chur the same day.

Ok, here’s my recommendation for a big tour (possible just Saturday or Sunday, but a good idea if the weather is better in Ticino than in Chur which is often the case and you need a swiss travel pass and no Eurail pass):

08.08 Chur Postautostation (bus)

10.20 Bellinzona Stazione

Chur–Bellinzona route - chur-bellinzona-route - PostBus (postauto.ch)

10.35 Bellinzona (Intercity train)

10.48 Lugano

Lugano | Switzerland Tourism (myswitzerland.com)

11.15 / 12.15 / 13.15 Lugano Centrale (boat)

11.50 / 13.00 / 13.50 Gandria

Gandria | Switzerland Tourism (myswitzerland.com) (recommended visit)

11.55 / 13.05 / 13.55 Gandria

12.40 / 13.30 / 14.40 Lugano Centrale (lake)

Lake Lugano Navigation Company - HOMEPAGE - Official Site - Società Navigazione del Lago di Lugano

15.31 Lugano Stazione Nord (Postauto “Palm Express”; needs to be reserved)

18.56 St. Moritz Bahnhof

Palm Express - palm-express - PostBus (postauto.ch)

19.02 / 20.02 / 21.02 St. Moritz (train)

21.04 / 22.09 / 23.10 Chur

Rail routes - Rhaetian Railway RhB