Jiri - the nearest roadhead for trekkers heading to Everest Base Camp
Jomsom - airstrip in the upper Kaligandaki valley. Most basic services can be found here; end (or start) of the Jomsom Trek. Ghasa to Jomsom Bus travel is also available (NRs 300 for Nepali and NRs 600 for foreigners), but you will have a heaven images in trekking
Trekking is the most popular activity, with a wide selection of possibilities, from deserts to jungles. It's also popular to study Yoga or Meditation. White Water Rafting is popular in many places.
Altitude sickness is a worry, with many of the passes in the Himalaya being over 5000m. Increase your elevation as slowly as possible, avoid flying from a low elevation to a high one, and physical activity; and drink lots of liquids after gaining altitude. Altitude sickness is unpredictable, and may strike people who haven't had problems before. Give yourself lots of flexibility in your plans, to avoid pushing yourself higher when you need to rest.
Stay up to date with the news, and be willing to change your plans, when going to places such as Kashmir or Nepal, that are facing armed uprisings.
Traffic on the narrow roads is often frightening, but due to the slow speeds is unlikely to result in fatalities.
Flights out of the Himalayas are often cancelled due to bad weather, be sure to give yourself at least a few days before needing to catch a connecting flight.