Well, nobodys mentioned the two premier arboreta of the US, namely Longwood Gardens , west of Philadelphia(1500 acres of evry kind of tree and plant arranged in forests , swamps, high deserts etc) and the National Arboretum off NY Avenue in DC. Both take multi days to take in.
Longwood has some neat programs for summer attractions.
1They have a Gilbert and Sullivan Festival wherein you can see some of the best interpretations by the Savoy Company and you can also get coached on some of the great G/S lines for use in cocktail banter and typical yupster "oneuppity talk"
2They show really neat fireworks at Longwood and nearby HAgley Gardens and Museum. The fireworks are choreographed to music and are mixed in with fountain displays, also choreographed to music. Last year they did an all HAndel ( not a great challenge) but still very impressive.
3 There are huge research facilities at Longwood and the Nat Arboretum. So if your a budding arborist, you can get tips , help, directions, criticism, and pity from really savvy people. Longwood and Winterthur are combined as a conservation, art prep, arbo-sciences, etc graduate school under the programs of the University of Delaware. They offer advanced degree programs in all sorts of arcane plantology and art and antique conservation and restoration.
I dont know anything else about these places except the websites
and unfortunately Ive used the quick reply so I dont think these will show up as a hot link, Well here goes anyway.
http://www.longwoodgardens.org
http://www.usna.usda.gov
WHOA DUDE_IT WORKED (Im easily impressed by gadgets that do things without being told)