Of course the king of ancient temples is Cambodia, but you can also see plenty of ancient sites in Thailand which also has many large national parks with plenty of nature to see. And of course there are several elephant experience places in Thailand as well. With that many (few) days I would consider just staying in Thailand. Maybe head up to Sukhothai for the old temple sites there. And there are big national parks nearby that area too with interest rock formations and plenty of jungle/forest. Or head east to Isaan. Maybe visit Phimai and Phnom Rung for the old temples. There's the massive Khao Yai national park near Khorat with waterfalls, meadows, forest, etc. Chiang Mai and Chiang Rai and that whole northern part of the country also has great parks and mountains plus some elephant camps.

I suppose time of year might make a slight difference as well.