It's National Park Week! Where to get your green on in 2011.

Every week in the US is a national park week, but only this week (April 16-24) is National Park Week, when all 394 of the nation's great natural and historic attractions are free to see.

The National Park Service (NPS) is also giving away prizes this week (including a trip to Shenandoah National Park, plus Lonely Planet guidebooks and the annual $80 park pass).

Here are a few ideas to aid your national-park adventures this year:

Shenandoah National Park

The National Park Service is offering a free-trip giveaway to this super Virginia park, which celebrates its 75th birthday this year. It's home to one of the nation's more fabled scenic drives: the Skyline Drive passes 75 overlooks into the Shenandoah valley. While in the area, splurge on a meal at the famous Inn at Little Washington or get campy at Dinosaur Land's forest of 50 life-sized dinos. (Our USA Trips guide includes several Virginia road trips, including this drive.)

Civil War turns 150!

Speaking of the south, get ready for four years' worth of Civil War festivals, lectures, reenactments and parties. Last week marked the 150th anniversary of the first shots of the US Civil War at Fort Sumter, South Carolina. The NPS has put together a Civil War site to keep track of all the blue-and-gray events at 70-some national parks. Our favorite is the moving Gettysburg National Military Park; the most memorable thing to do is watching dusk from Little Round Top, with views of the battlefield.

And, if you've ever wondered how reenactors decide who dies first, watch this video.

Las Vegas: USA's greatest national park hub?

Utah's Zion National Park apparently sells more annual passes than any other. Makes sense. It's the closest to Vegas, which is a highly underrated gateway to the country's greatest national parks (there's over a dozen within half-a-day's drive).

So! As we recently told to ABC News Now, use Vegas (with its cheap flights and car rental) for your national parks adventures. To the east, Utah's parks offer unexpected colors and rugged landscapes, south is Grand Canyon (here's how you, or the Obamas, can beat the crowds); Nevada and California have a flurry of options too, including Joshua Tree, which celebrates two birthdays in 2011: 75 for the park, and 25 since U2 came by for the photo shoot of their 1986 album Joshua Tree. You can stay at the same hotel, the Harmony Motel, for about $70 in nearby 29 Palms.

The youngest national park

Opened in 2007, the Sand Creek Massacre National Historic Site, in eastern Colorado, highlights a bit of history not commonly known: an 1864 raid on Arapaho and Cheyenne Indians, resulting in the death of 150 people, mostly women and children. Guides can interpret the area, mostly open horizon in the Colorado plains.

Delaware: pick the state's first national park

Dare we suggest going to the only state without a national park? Perhaps this is an occasion to settle your own mind on what site best warrants to be the first national park in the USA's first state. Perhaps Historic New Castle, on lovely Rte 9 on the Delaware River? Or we're also fond of the lesser-known 'Eastern Delaware,' a chunk of New Jersey accidentally allotted to Delaware.

Note: If you can't make it to one this week, other free-entry days this year include the first day of summer (June 21), Public Lands Day (September 24) and Veteran's Day weekend (November 11-13).