Thanks for the information.

I am not specifically after just trekking (although that is enjoyable) but day trips out of Jakarta and into the countryside. If I could hire a car there then I think it would be easy, but from what I have read it is not recommended to drive in Indonesia, thus thinking better to have a guide/driver and also for local knowledge. Here in the UK it is very easy to take a train directly to a national park or similar and get straight on the walks and explore rural regions and then take a return train home. e.g. London to the South Downs, Manchester to the Lake District etc.