It's one of the quirks budget-conscious travelers appreciate about the National Parks: Even though demand goes up, prices can stay the same.

Last year, more than 285 million people visited properties of the National Park Service, up about 4% from 2008, according to the government agency. But the cost of an annual pass that gets you into all of the lands managed by the Park Service, as well as the U.S. Fish and Wildlife Service is $80 this year, same as last. Over the course of 2010, the Park Service will waive entrance fees a handful of times just as it did last year. And while some parks may raise their $5- to $25-per-carload fee, many are holding steady, and as always, more than 200 are free. Camping is definitely one of the more inexpensive vacations you can take, says Heather Hunter, a spokeswoman for AAA.

In honor of National Park Week (April 17-25), all the national parks are waiving entrance fees this week. That saves vacationers $15 per car at Joshua Tree National Park in Southern California, and $25 per carload at Grand Canyon National Park. Some hotels near national parks have also slashed prices for the week. The Rusty Parrot in Jackson, Wyo. near both Yellowstone and Grand Teton national parks has rates starting at $157 a night instead of the usual $230, a savings of 32%.

Of course, the downside is that the parks could be fairly crowded during the peak summer season, with longer entrance lines, more hikers on trails and more campers competing for places to pitch their tents. But if you can put up with the congestion, there are deals to be had. Here's how to track some down for this weekend -- and throughout the rest of the year:

Look for park promotions

Nationwide, park fees are waived several times a year, says Mark Shields, a spokesman for the National Park Foundation, the government-created nonprofit that works with the National Park Service. In addition to National Park Week this spring, fees will be waived on Sept. 25 for Public Lands Day, and Nov. 11 for Veterans Day. Individual parks host free weekends, too.

Check for lodging deals

If camping isn't your style, check hotels and other lodging options nearby, Hunter says. Many offer deals tied to the national parks, especially if there's a free park weekend going on. Through April 29, the National Park Inn near Mount Rainier National Park in Washington, is offering two weekday nights for the price of one. Rates start at $111. Skyland Resort outside Shenandoah National Park in Virginia is also offering a two-for-one deal through April 29, with rates starting at $125.

Buy a pass

An annual pass to a single national park can pay for itself in as few as two visits. One seven-day vehicle pass to Big Bend National Park in Texas costs $20; a pass that lasts 12 months from your date of purchase is $40.

For families planning to visit more than one park over the year, there is an $80 interagency annual pass, which offers free entry to lands managed by the National Park Service, U.S. Fish and Wildlife Service, and others for 12 months after purchase. One pass covers accompanying passengers in the vehicle, including all children ages 15 or younger and up to three adults, Shields says. Tally up park admission rates to see if you'll come out ahead.

Bring a senior

Anyone 62 or older is eligible for the $10 lifetime interagency pass, says Joan Rattner Heilman, author of Unbelievably Good Deals and Great Adventures That You Absolutely Can't Get Unless You're Over 50. The pass is good at all the same properties an annual pass covers, and still extends to accompanying passengers in your vehicle. Senior passes also include a 50% discount on amenity fees for camping, boat launching and other services.

Time your visit

Summer is prime tourist season, but travelers should base timing on what they'd like to do in the park. There are a lot of spectacular parks this time of year because the wildflowers are out and about, says Kurt Repanshek, the editor of NationalParksTraveler.com, a travel advice site. Right now, Death Valley National Park in California and Nevada has daily temperatures in the 80s and is at peak wildflower bloom.

Parks and lodging may also be less expensive during shoulder season. For example, Glacier National Park in Montana charges $25 per carload between May 1 and Nov. 30, and $15 between Dec. 1 and April 30.

Reserve in advance

Like any other tourist destination, national parks fill up fast during their peak seasons. Make reservations for camps or nearby lodging well in advance, Hunter says. This might take multiple phone calls. Tour groups often book large blocks based on anticipated traffic and then scale back once they have exact numbers, says Repanshek of NationalParksTraveler.com.