There are routes for all kinds of bikers near Jakarta.
MTB riders have plenty of choice, with some challenging hills and scenic rides near Sentul and longer rides near Puncak Pass and further afield. GPS is a good idea for navigation - if you have a Garmin, others in the group have waypoints and routes available
Road/triathlon riders will find things a bit more limited, but the roads near Sentul city are good for training (around 14km per loop) and there are longer road rides, up to 100+km which are suitable for road bikes (although some less maintained sections of road would be better for hybrid/cyclocross bikes)
Touring cyclists will enjoy the quieter roads further away from Jakarta
Those looking for a gentle ride with the family can try Jalan Sudirman in the centre of Jakarta when it's closed to cars on a Sunday morning once a month
Nocturnal cyclists will enjoy the Wednesday evening MTB rides - good lights or plenty of carrots needed!