I was perusing the Mohegan Sun site ('cuz I love planning weddings! LOL) and their local things to do page gave me some ideas.
To get around the problem of having your ceremony at one place and reception at another IF there is a weather issue how about having a ceremony and cocktail party at one place...and THEN returning to Mohegan?
You can see if one of the local country clubs/golf clubs is willing to let you get married there. They generally have beautiful grounds and whatever space you'll have a small cocktail hour at can be used for backup.
Mohegan Sun Country Club at Pautipaug
(Baltic, CT)
860.822.6375
mohegansun.com/golf
Connecticut Golf Courses
ctgolfer.com
Elmridge Golf Course
(Pawcatuck, CT)
860.599.2248
elmridgegolf.com
Fox Hopyard Golf Club
(East Haddam, CT)
860.434.6644
golfthefox.com
Norwich Golf Course
(Norwich, CT)
860.889.6973
norwichgolf.com
River Ridge Golf Course
(Griswold, CT)
860.376.3268
riverridgegolf.com
Museums/Attractions
Another thought was a museum. Many have nice grounds, and of course, all have indoor spaces. They might not require you to have any part of your reception there, I'm thinking they might allow a small 25 person ceremony for good donation.
Lyman Allyn Art Museum
(New London, CT)
860.443.2545
lymanallyn.org
Mystic Seaport - The Museum of America and the Sea
(Mystic, CT)
860.572.0711
mysticseaport.org
Denison Pequotsepos Nature Center
(Mystic, CT)
860.536.1216
dpnc.org
Gillette Castle State Park
(Haddam, CT)
860.526.2336
ct.gov/dep/cwp/view.asp?A=2716&Q=325204
Another thought along the cocktail hour idea was a vineyard/winery. Lovely, lovely grounds, all will have some sort of indoor private space in case of rain, and then work out a cocktail hour or wine tasting.
Connecticut Wine Trail
(Statewide)
860.677.5467
ctwine.com
Chamard Vineyards
(Clinton, CT)
860.664.0299
chamard.com
Jonathan Edwards Winery
(North Stonington, CT)
860.535.0202
jedwardswinery.com
Stonington Vineyards
(Stonington, CT)
860.535.1222
stoningtonvineyards.com