Secluded cave near Lagoa, Algarve, Portugal [Portugal is has it all. great food and wine, sights, natural spots (like this one), friendly people... and you can get away with your high school Spanish if you need to. jh]
Dark cave, 15km South of Phong Nha, Quang Binh Province, Middle Vietnam // more of a fun half-day out than a proper cave tour: ziplining, swimming, mud bath, kayaking // price: ~270,000? // You won’t need anything other than swimwear and maybe your go pro for this trip. The mud does stain anything that is white. If you are bringing any valuables, be sure to either go with a tour or a lockable vehicle, as their lockers have a bad reputation.