In Vietnam every hotel will have a “tours desk” that will happily book a tour or visit to any local sight imaginable. There are also dozens of independent travel agencies around booking these same tours, but some of them are better than others. I prefer to book tours through my hotel, even if it costs a bit more, because it helps them and they have a strong incentive for you to be happy with the tour. If a hotel would consistently book guests on tours that the guests felt were rip-offs, that would be in the reviews and they would lose future business. More expensive hotels tend to charge more for the same tours, but things in Vietnam are so cheap that it’s not worth worrying about. And you might get a better pick-up at the nicer hotels as well.
Southeast Asia is easier to deal with, and there are cheap flights connecting in the Middle East and landing in Bangkok, Kuala Lumpur, and Singapore. Each of those is a very interesting city, and there are many smaller places to go in the same region. In Malaysia there is Penang and Melaka. In Thailand there is Chiang Mai and quite a few others in that area. In Laos you can visit Luang Prabang, which sounds like it could be a winner for you. And there is also Siem Reap, which is home to the amazing Angkor Wat temple complex.