El Malpais National Monument and El Morro National Monument (known as ELMA & ELMO, if you're familiar with NPS Alpha Code lingo) are jointly worth a visit if you find yourself in southeastern NM.

El Malpais means "the badlands," while El Morro means "the headland," both given their name by Spanish explorers in the 19th century.

El Malpais gets its name from the fact that the monument preserves a vast volcanic field on the southeast corner of the Colorado Plateau. The photos here are from Sandstone Bluffs Overlook. Much of this area was used as a practice bombing range during WWII, and there are signs everywhere warning of the possibility of UXO. There are a couple of lava tubes you can explore (requires a permit from the VC). I didn't go in those, however, because of the sign you see in my post. The road to the tubes is an unimproved, and largely unmaintained dirt..."road" with the ominous sign you see here. It had rained a few days prior to my visit, and the rangers suggested I not try to go down the road on that day. I took their advice. lol

El Morro gets its name from the massive rock promontory you see in the photos, upon which an ancient 350+ room pueblo sits. There is a massive spring at the base of the promontory. The rock and the spring served many people traveling through the area long after the pueblo was abandoned. Many of those travelers left inscriptions on the rocks, some of which are still visible to this day. 

The monuments are about 20 miles apart and are jointly managed by the same staff. They're right off I-40 outside Grants, NM.

El Malpais NM & El Morro NM