Going in circles

_{This is part 11 of the puzzle series that started at Living the traveller’s dream. Each part is solvable on its own.}

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Dear Puzzling,

Don’t ask me how or why, but this week I have ended up at almost the exact same place as I was a few weeks ago. Let’s just say that some places are worth visiting more than once in a single trip? It’s definitely not the case that my overall travel plan makes no sense when looked at from an actual traveller’s perspective. Nope, that’s not it at all.

Each puzzle has five "thematic" answers, which are geographical locations all in the same country. Each is notable enough to have its own article in the English Wikipedia, although not necessarily under the same name I use. Your task is to fill the grid, highlight the thematic entries, and guess which country I am currently visiting (which is not indicated in the grid). Some of the answers will be needed in the final part of the series. Except for the last part, all others are solvable on their own without knowledge of the previous puzzles.

This week I have seen massive amounts of water pass through an impressive man-made barrier, toured a large forested highland area, strolled in an idyllic urban park, and seen the country’s tallest skyscraper. Can you guess where I am?

Wish you were here!
Love, Gladys.

Across
6. Content of plastic cup’s evenly flipped for explorer (8)
8. Bodily tissue man found near Colorado, in America (6)
10. Over the air, look at dock (4)
11. I put a maid out to show embankment (6,3)
13. Skyscraper’s not heartless proprietor standing behind Gladys and company (5,5)
15. Assist with a Verdi work (4)
16. A pet worked longer than normal hours to produce piece of kitchenware (6)
17. Novelty sports car containing, say, American prosecutor that’s retired (6)
20. Categories finally put one by one (4)
22. Balance energy in ultra-tiny bananas (10)
23. Resourceful inside information linked to debts (9)
25. Somewhat adversarial garment from the south of Asia (4)
27. With a bit of legalese involved, create part of legal document (6)
28. Sylvester Stallone has vandalised Crete in a furtive manner (8)

Down
1. Mean mice rode bats (8)
2. Pilot is the first to go up in Klondike (3)
3. Part of NYC museum’s wall I would be in possession of after accepting debt, finally (7)
4. Military group of two captured politician (4)
5. (See 12 down)
7. Pam and Raul miss finals having a nap after allocated time to describe highland area (6,7)
9. Father with key let into cutesie, artsy city (6,3,4)
12/5. Part of NYC gallery near Manhattan’s southern edge is left covered in sand (6,6)
14. Notice old spear piercing communist (6)
18. Forest dweller I have confidence in, 100% (8)
19. American with a firearm, flip-flops seen around university park (2,5)
21. First of names held by a Republican of advanced age? (6)
24. Climber to carefully observe German beer broadcast (4)
26. Carry out truth, getting rid of falsehood at first (3)

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Gladys will return in Rich and unique.