Weekend short trips (Istanbul)

Hi,
Who has good tips for an "Istanbul weekend run away location". Nice spots around Istanbul that can be reached within 3 or 4 hours and are worth a visit. Of main interest to me would be spots away from traffic and chaos, with lots of nature :-) or of cultural interest which are not necessarily printed in a travel guide. Still do not hesitate to post everything you consider of interest or worth a visit.
Jan