To visit the Royal Tombs and the Pagodas along the Perfume River there are two options. One is booking one of the boat tours. The other is renting a car (or a motorbike, for the more adventurous).
The boat tour is slower, but more beautiful. Royal Tombs are quite far from the river. You will have to rent a motorbike taxi at almost every stop. It won't be difficult to find a bike, taxi drivers will be waiting for you. You will have to add the cost of the motorbikes and the entry tickets to the tour price. This may more than double the price of the tour offered in tourist offices. But it is money well spent. If you do the boat tour, do visit all the temples and tombs.
The other option to visit the Royal Tombs, is renting a taxi. This is a cheaper and more convenient option. But less beautiful. The taxi will take you to all the tombs, and wait till you finish the visit. You should book a private tour.