It's impossible to give a simple answer to this. The United States is as big as Brazil. Most of it is crossed by good roads and there are dozens of national parks, and hundreds and hundreds of state parks. Many cities have lovely city parks. You can probably find something nice in any of the fifty states.
One suggestion I have is that you just might enjoy a "college town," particularly a city with a big state university. Such towns always have pleasant, inexpensive restaurants and coffee shops, and students know how to live cheaply, and some of them like outdoor outings and can give suggestions.
It is, honestly, difficult getting around in the US if you don't have a car, and renting cars is expensive. So my other idea is that you might want to choose a city that has good public transportation, and that somewhat limits your choices.
I live near Boston so I can just tell you about Boston. Boston is not the #1 tourist city, but, yes, it's one (of many) good cities to visit.
Boston has good public transportation and is a good "walking town." The "Freedom Trail" is a path--it's actually a red line marked by red bricks in the sidewalk--that is 4 km. long and connects 18 historical sites. It's relatively inexpensive, and winds through places in downtown Boston where you can find places to eat. It winds through the North End which has wonderful Italian restaurants.
One of the sites on the Freedom Trail is the USS Constitution, the oldest ship in the US Navy and theoretically still in service, which fought in the War of 1812. It's very interesting to visit.
Boston has a nice park, "the Esplanade," on the banks of the Charles River.
The Boston Inner Harbor Ferry is a cheap way to get a 15-minute boat ride across Boston Harbor.
You can book whale watching cruises that leave from the New England Aquarium.
Dan Smith has a great point about college towns. They tend to be small, friendly, and have great coffee houses and good public transportation.
Boston is an excellent idea, for all the reasons Dan mentioned. Plus, the Boston has a few Brazilian-American (and Portuguese-American) communities in case you get homesick, want some familiar food, or just need a break from speaking English all the time.
Another great place might be Portland, Oregon. It's smaller than Seattle, and has better public transportation; but shares Seattle's advantages of friendly people, great coffeehouses, and proximity to spectacular scenery.
(One thing about both Portland and Seattle: it does rain a lot, and can get quite cold in the winter. July and August are the best months for sunshine.)
Wherever you decide to go, I hope you have a wonderful time! :)