If I wanted to go to a national park in late March or April, what would everyone recommend? I'd like to be able to comfortably hike around outside -- so highs in the 60's or 70's would be ideal temperatures but I am open to a little cooler.

Joshua Tree or Death Valley in California. There's also Anza Borrego which is a state park.

Everglades?

Joshua Tree and Death Valley are very nice but I vote for Big Bend NP. Fantastic hiking!Much cooler in the morning than you want. Death Valley and Joshua can also have very cool nights and mornings. In the southeast corner of AZ there is a less popular Chiricahua National Monument. Another park great for hiking this time of the year, IMHO, is Saguaro NP

Hard to pick just one as they are so different. My favorite is Bryce.

U.S. Virgin Islands National Park would lovely that time of year!

Early April is our favorite time for hiking at the Grand Canyon. The early mornings and evenings are quite cool (almost freezing), but at mid-day we wear shorts.

I have also been to Yosemite Valley at that time and enjoyed beautiful 60-70 degree weather and great hiking. I don't know if that was typical or unusual for that time of year there.

I've been to Yosemite Valley in early April with snow. Hard to say if you would get that, but I think Enzian's warmer experience is pretty unusual that time of year. I'd vote for a Joshua Tree/ Anza Borrego trip, which we did that time of year twice (chilly mornings with warm afternoons).

Hawaii Volcanoes National Park

Statue of Liberty

Grand Canyon

Big Bend
Death Valley
Everglades/ Key Biscayne/Dry Tortuga

Grand Canyon and most of the Utah Parks you would need to check and decide a couple of days before you were going at that time of year to make sure weather was cooperating. Bryce in Utah would be especially worrisome at that time. If you are wanting to do a lot of hiking consider Big Bend and Death Valley.

Ooh! Dry Tortugas is an excellent idea!

Bill Baggs is a state park but the beaches there are stunning.

Saguaro or Big Bend - I've been to both during those months... perfect! Just be sure to avoid Big Bed during Texas' "spring break," or the trails will be full of families with children!