Hiking in autumn in Romania is definitely a great experience. Starting with late September the leaves start to turn and the nature puts on a great show! There are many good destinations for hiking in Romania, but one of the best is Oltenia from under the mountain, with highlights that include Parang Mountains, Buila Vanturarita National Park, the old kulas (manors) near Horezu, and much more.
If you want to do hiking in Romania in autumn, the best time to see the beautiful autumn colors is September 20 till October 25. That is when the colors seem to explode and one can see trees colored in green, yellow, brown, red.
As the weather gets cooler, one needs warm clothes for hiking, good hiking boots, and a good appetite for cold and wind :). Photography in the mountains will definitely be the best part of your trip in Romania, in autumn.