You don't have that dilemma in Laos. That's easy. For the sea you need to visit our neighbours. China, Cambodia, Myanmar, Thailand or Vietnam. You can find a lot of spectacular karst mountains in Laos though. Photo above: Hin Boun river (Bolikhamsai province).
Take route 13 South from Vientiane. Turn left at route 8 or 12 in the direction of Vietnam. Photo above: the road from Mahaxay to Thakhek (Khammuan province).
Where there is karst, you can find caves. Photo above: Hin Boun river seen from Sala Hin Boun guesthouse. From here it's a 45 minutes boat ride to Kong Lor cave.
Another small boat will manoeuvre you through the 7 kilometre long cave. Claustrophobic tourists might ask themselves why they didn't stay at the sea...