How long do you plan to be in Death Valley, and what kinds of experiences are you interested in? For most visitors, a regular economy or midsized car is adequate, but if you want to spend a little more time and get into some of the moderately backcountry areas, a 4wd and/or high clearance is helpful, especially in December when there will be rain around the park and some snow at higher elevations.
There are some long, gradual grades where some people have trouble because they have no experience shifting gears and it doesn't come naturally; if you make full use of your transmission (whether automatic or manual) and shift down before beginning a long ascent, any car will be fine. (Ditto for long downgrades; shift down and let the reduced engine speed slow you down, rather than overheating your brakes).
The Jeep rental company is Farabee’s, at Furnace Creek Inn. Their Wranglers are fully outfitted for backcountry use, and with your rental they provide maps, trip ideas, and an ice chest. They start at $175 a day and rentals are by the day only, so you couldn’t get one for an extended camping trip. But it works for folks who don’t want the cost of renting a 4wd in Las Vegas when they only want it for short explorations in Death Valley—and unlike city rental companies, they put heavy-duty tires on the vehicles and expect you to go and play in the dirt.
Sutter & Jes are right, you won’t need a special vehicle to see many of the natural marvels and historic sites. The main roads through the park are all paved: Hwy 190 (a state highway), Badwater Road, Daylight Pass, Mud Canyon, Beatty Cutoff, North Highway (aka Castle Road), and Wildrose-Emigrant Road. Wildrose-Emigrant does have a 3-mile stretch that’s graded gravel, because it won’t stay paved; flashflooding and runoff from a spring kept tearing it up, and around 1987 the NPS stopped paving it and just maintains it regularly. Of the main entry roads, this is also the only one with a length limit, 25’ and under, because it is steep, narrow, and curvy in places. Otherwise, it’s fine for any car driven carefully. There are plans to rebuild it and reroute it to remedy the ongoing flooding issues, but that won’t get rid of the steep twisty parts.
Devil’s Golf Course, Natural Bridge, Mosaic Canyon, and Wildrose Charcoal Kilns are on graded gravel roads that are maintained often, because these are major visitor sites. Golden Canyon does not require a drive; you park just off Badwater Road and then walk into the canyon. Artist’s Drive (paved) and 20 Mule Team Canyon (hard dirt and clay) have 25’ length limits because of sharp dips; the top ¼ mile of Dante’s View Road has the same limit because it is steep with hairpin curves; there is parking for oversize vehicles just before that point. It sounds like you plan to rent a regular car, so size won’t be an issue. Ubehebe Crater is on a paved road. For graded gravel roads, take it easy mainly to prevent flat tires; you won’t be able to drive a car comfortably at any speed fast enough to harm the suspension.
Of all the backcountry trips that first-time visitors would be interested in, I think Titus Canyon is the most rewarding, a one-way (east to west only) trip of 26 miles in a double-ended canyon with scads of things to see: higher elevation plant life, colorful rock formations, a ghost town (founded on a mining scam), petroglyphs, a chance to see bighorn sheep if you go early in the morning, and sheer, narrow walls at the west end that show the power of flashfloods in the desert. It is not an extremely rigorous trip and there is plenty for everyone’s interests; however you cannot change your mind halfway in and go back. This should not usually be attempted in a regular car.
December is winter in Death Valley. Temperatures are generally fairly moderate, but it can rain quite a lot on the Valley floor and up to about 4500-5000’ in elevation. Above that, snow is possible. Some of the heaviest rain I have ever seen in California was in Death Valley, at Stovepipe Wells (sea level). Some of us TAs have been there when almost every secondary highway or spur road was closed because of rain damage, icy surface, or accumulated snow. I've seen enough accumulated snow on Wildrose Road to make 4wd necessary and narrow the useable surface to one vehicle width. These conditions are not typical, but they ARE possible, so before setting out for your day’s sightseeing, check with the Park Service at a visitor center or ranger station, or if you’re staying in park lodging, ask at your hotel desk.
Have fun! Death Valley is the most awesome place you can imagine, and if you can give yourself 2-3 days to enjoy it in this comfortable weather, you’ll never forget it! Browse through some of the recent threads and maybe they’ll give you some ideas about things to do. If you want more details or have some specific interests, we can help you.