When going to Tangkuban Perahu by rental car, ask to pay the entrance fee yourself. Don't let the driver do this for u as this gives him a chance to overcharge u. And don't forget to ask for the ticket. Bandung is a lovely place to visit but it's those dishonest behaviour that could spoil your trip. (It spoilt mine!)
Initially know as the engaged tourist guide (they have tags endorsed by the park), but you will be followed by one and then few will tag along. They will follow you to the ciater, which is located on the other end. The trap comes once they follow you there to the ciater, they will try to sell you some soverneirs at exhorbitant price and will try to tell you that they can't accept cash as tips but they need to sell us some items (key chains, pens) in order to accept the so called tips.
Unique Suggestions: Tell them you already have your own guide or engaged one provided or recommended by your agent (if you need some guide explanation).
Fun Alternatives: Just go to the information counter and ask for the direction to the ciater and go down there by yourself.
But beware as the road down the ciater is very quiet as not many people go down and up so frequent.
When you reach the top of the mountain, overlooking Kawah Ratu, there'll be some people wearing the vest written "Guide" who would approach you, offering to take you to Kawah Domas, another crater within walking distance. The trek to Kawah Domas will take about half hour or so, depending on your fitness level.
Anyway, before you agree to be taken to the crater, make sure you agree with the fee. Apparently, the fee is not standardized...they could charge you by individual or by group.
There would also be people selling souvenirs, who would followed you during the trek and would pester you to buy from them, when you reach the bottom.
Unique Suggestions: If you need to agree on the guide fee and don't speak Bahasa Indonesia, ask your driver to speak on your behalf. To agree upfront is a better option, rather than paying them ridiculously when you reach your final destination.
Tangkuban Perahu is the nearest active volcano in Bandung. You can reach this place about 30 minutes by car.