Downton Abbey

I'll be direct here: Go see the "Costumes of Downton Abbey" exhibit at Wintherthur. It is absolutely marvelous.
You get to see, close up, a large selection of the costumes worn in the show. From the upstairs crowd, there are exquisitely beaded and embroidered evening dresses and wraps, perfectly tailored suits, walking outfits, gauzy summer frocks, impeccably correct cricketing togs, the American tycoon Carlisle's ill-chosen weekend wear (he wore shooting clothes instead of walking clothes, WHAT a faux pas!), Lady Edith's wedding dress (it had to be strong enough to weather the drama of that day), mauve mourning dresses, Lady Sybil's daring harem pants outfit -- and oh my, the beautiful hats! You also get to see one of O'Brien's lady's maid outfits, Mr. Carson's suit, Anna's crisp white aprons, trimmed with lace, and Mrs. Patmore's practical purple kitchen dress.

The show also compares and contrasts the Granthams and the Crawleys with the Duponts, who led an upper-class American lifestyle at Winterthur during the same time period. The exhibit gives details about the Duponts' servants and shows Mr. Dupont's capacious traveling trunk and Mrs. Dupont's custom-made fitted cosmetic case (with an early version of a curling iron).
The tour guides were wonderful, sharing interesting nuggets of information over and above the ample material given in the displays. There are also video clips from the show, lines from the script, a box of smoky tea to smell, a swatch of super-luxury vicuna fabric to feel, and behind-the-scenes explanations from the show's costume designer about how her department used vintage embroidery and fabric to create some of the clothes.
Of course there's a gift shop of souvenirs as you leave the exhibit. You can buy books about the fashions and customs of the era, tea with photos of DA characters on the canister label, fancy hats, lace gloves, T-shirts, barrettes, costume jewelry and tiaras, napkins, tea towels, pillows, jigsaw puzzles, toiletries, CDs, videos and even cleaning products (for your staff at home to use). I would've bought a knockoff of one of Anna's aprons, but I didn't see one.
To get to the exhibit from the visitors' center you can take a shuttle bus or walk; the trips take about the same amount of time. I enjoyed the well-marked walk very much: I got to see the old greenhouses and the peony garden, which was in full bloom.
You need to buy a timed ticket for the Downton Abbey exhibit. You can buy it online, but once you're at the museum you have to check in and exchange your receipt for an actual physical ticket -- which I found out only after waiting for 10 minutes in what turned out to be the wrong line anyway. Also, watch where you park in the visitors' center lot: not all the handicapped spots are marked individually, so I accidentally parked in one, which I hate doing; I feel like going around apologizing to anyone who might have seen me.
And just a warning for visitors who haven't seen all four seasons: there are spoilers in the exhibit.