Mexico honeymoon recommendations

Hi everyone! I'm hoping to get recommendations for the second part of our honeymoon. We're pretty set on going to Tulum, and while I'm sure we could find enough to do there for two weeks, we want to split up the locations.

I've been trying to get more info on the general vibe of popular Mexico tourist areas and haven't seen much! Of course Cancun is a party area, that's not really our vibe. We chose Tulum because it's gorgeous, close to us (Arizona), has lots of physical activities and historic attractions (we've always wanted to see Mayan pyramids), has nice weather in December, and is somewhat affordable (total honeymoon budget is around $8k).

For the second half, I was thinking more of an urban area could be fun to explore. I've heard great things about Merida, and I also like the idea of Mexico City, but I'm not sure how "romantic" it is. Please give me any tips or recs you have!