Hands down one of the best hikes in Palestine involves walking along the Wadi Quelt - what some believe to be the biblical Valley of Shadows.
This hike is not necessarily hard, but it is very long, and it can get hot in summer, so the best time to go is in Spring between March and mid-May - although there are certainly possibilities to go during the warmer summer months, as long as you pack more water, sunscreen and start early.
What I love about this hike is the variety - walking along a river, past an oasis, swimming holes, shepherds, monasteries, as well as extremely dry patches of countryside. The earlier you go the higher the chances are of catching a glimpse of the animals that live near the river.
The part of the walk that I recommend starts at Ein-Fawar, just over half way between Ramallah and Jerich, and about 30 mins by Taxi from Ramallah. A taxi there will cost around $20. We left Ramallah at 6.30am in late May to do this hike, finished it around 2pm and were very exhausted by the end!
More info in Steven Szepsi's excellent book on walking in Palestine.