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.
Bogor is reachable by train and bus.
And it's sufficiently different to the South Downs to make it interesting to you
You could always stay there overnight - there are some nice hotels, and a stay makes it easier to relax and enjoy the walk around the gardens.
I rate Bandung and its surrounds. But a day trip would be pointless. And avoid weekends when half Jakarta seems to go there, and good part of KL too!