I visited Bhutan in December 2017 and hence thought of sharing some tips that will come handy if you planning a trip to Bhutan in near future. So will directly get to the point..
First and foremost thing to remember is that there are only two airlines that operate in Bhutan - Bhutan airlines and Druk airlines. At times you don't see the flights to Bhutan on an aggregator website hence it is suggested that you visit the websites of the these airlines and book your tickets.
Second, Once you have booked your flight tickets be very wise while doing a web check-in and if possible grab a seat that offers you the Himalayas view. For example traveling from Kolkata, India to Bhutan the Himalayas can be seen on your left hand side. You will be able to witness K2, Mt. Everest and other mountain peaks too. So research the route and opt for your seats.
Third, While you are planning your itinerary for the trip make sure that Tiger's Nest trek is planned on the last day. As it is super tiring and you would not want to have it in the initial part of your schedule
Fourth, It is very easy to get a mobile connection in Bhutan.. connection preferred would be B mobile, you just need your passport to get the connection. You can recharge the mobile with 99 Rs and avail a very good connectivity throughout Bhutan. including 4G at Dochula pass... :)
Fifth, The most beautiful part of your trip is going to be the wonderful architecture of various monasteries in Bhutan. But unfortunately clicking pictures is not allowed inside them. And hence the smartest way is to keep your mobile or camera aside and capture the beauty in your eyes that will remain in your memories for years to come.
Sixth, Though cities like Paro and Thimpu have many ATM machines but it is still advisable to carry some cash in emergency. As at times these ATM machines don't work and specially if the servers are down all the machines will have problem. Also a lot of places like local markets, Hotels and guides won't accept card for their payment. Hence do carry some cash with you and also withdraw some extra currency when you see any working ATM machines on the way. Currency that works in Bhutan are Indian Rupees(INR) and Bhutanese Ngultrum.
I wish you the best of experience in Bhutan..!!