There’s a temple on top of a mountain called Yamadera — it’s about an hour’s train ride outside of Sendai, and it’s easily a must-visit if you’re in the city.
Conveniently enough, three must-see Sendai sightseeing spots are all clustered together (they’re each about a fifteen minute walk apart), so it’s pretty easy to see them all in one shot.
Is there anybody in the history of the planet who went to Siem Reap but didn’t go to Angkor Wat? I suppose there was probably at least one maniac who did this at some point, but it certainly wasn’t me.
There are a ridiculous amount of temples in Chiang Mai. I don’t think you can walk more than a block or two without stumbling onto a temple. It reminds a bit of Kyoto in that way.
If you’re going to Borobudur Temple, you’re probably going to do Prambanan, too — it’s the other major temple in Yogyakarta, and another big reason why people come here in the first place. You can even buy a combined Prambanan/Borobudur ticket, so yeah, don’t fight it. You’re going to see both.
Saying “if you’re in Yogyakarta, you have to see Borobudur” is kind of like saying “if you’re in Paris, you have to see the Eiffel Tower.” Duh.
But seriously: if you’re in Yogyakarta, you have to see Borobudur. It’s amazing.