Tretes is a small town about an hour's drive from the second largest city in Indonesia - Surabaya, in East Java. It's on the slopes of Mt. Welirang. It's possible to take a day trip to Tretes from either Malang or Surabaya. But the traffic jams usually make the trip longer - especially on Sunday afternoons when everybody wants to go home to the cities at the same time from the mountain town of Tretes. Tretes has several small hotels, some with swimming pools, (heated or not). Besides staying in a hotel, you can also or rent a small bungalow for a visit.
Tretes has a much cooler climate, when compared to Surabaya. You can sleep without air conditioning and still be comfortable.