Once in a lifetime destinations - where do you recommend?

Hi, all!

My husband and I are planning on taking a "once in a lifetime" trip in September and are trying to decide where to go. For context, it will just be the 2 of us and we would ideally like to be gone for around 8-10 days. We went to Bora Bora for our honeymoon and it was.... absolutely once in a lifetime. We're trying to find another special place like that that we will likely never return to. Doesn't need to be beaches - we're open to adventure, cold weather destinations, etc. Our one idea right now is an African Safari, but we want to explore other ideas as well. Please share the most unique and special once in a lifetime trips you've ever taken!

Edit: wow didn’t expect this to get as many replies as it did! Thanks so much everybody for your amazing suggestions. Sorry I didn’t add as much detail as I should have. We are in the US and we’d prefer to spend under $15k USD. I didn’t add much detail about the type of trip because we truly have no preference and are pretty versatile - we love to hike, adventure, camp, explore cities, but we could also go to a relaxing beach destination and be happy. Truly just wanted to get everybody’s take on the most magical trip they’ve ever taken and you guys did not disappoint!