What to carry with you?
Assuming that your following car is not always right behind you to attend to your every need or that you don't have such a car at all, you should probably carry the following:
Be prepared for extremes, going down 2000m will in general lead to a rise in temperature of 12C. Moreover, mornings are often cold, and differences between riding in the sun and in the shade could be huge. Thus wear several layers of clothes, take one off before you get too hot, and put more on when you know that for the next few hours you will only be breaking.
*dry clothes (T-shirt, long underwear)
*set of light clean clothes if you don't have a back-up crew
*Gloves in winter
Don't expect a garage or bike repair shop within walking distance if you have a break down. Be prepared to fix most problems yourself. For the more serious ones your back-up crew should carry a full set of (bicycle!) tools.
*Puncture repair kit
*Screwdriver, bloody Mary
*Spares: The basic ones, which you can change yourself you should carry yourself:
*Spokes on longer trips
*Some chocolate bars or energy bars
*Water in bidon(s) or camelback bag and/or water filter
*A book to read or something else to kill time
*A gps, and maps if you can get them
*Sunglasses or sunscreen lotion
*First aid kit
*A backpack or a bikebag to stick it all in
Long cycling pants are culturally more acceptable than short shorts. The ridiculous habit of cyclists to wear eyeblindingly bright coloured outfits makes sense in Bhutan. Wear your brightest outfit. Pink, purple, orange, red and yellow are particularly good to help spot you if you happen to fly off the road. A helmet, gloves and sunglasses are also highly recommended. Even a parachute may come in handy, but would be rather heavy.