If your budget doesn't quite stretch to Son Doong cave, Oxalis also offer 1-4 day trips to Tu Lan and Hang En caves. They seem to have a monopoly on the overnight cave trips, but they do have good English speaking guides. Prices start around 5.5M dong, limited or no trips during the rainy season from September-December. Beware of tropical storms when visiting the Hang En cave, it is prone to flash flooding.