Death Valley is huge, you will need many days to see the "best of the best". We took the kids there when we first got our TM, they loved it. Furnace Creek has some full hookup sites, but I'm pretty sure that they're booked up for most of January. Do you have a tow vehicle? A lot of the roads are very narrow and one-way. I was glad we had our 4x4 pickup. DD had her learning permit and got to drive around a bit! Awww, she's all grown up now, but she will never forget that trip.
When we stayed there you could pay a nominal fee at the Furnace Creek Ranch hotel and use their showers & the pool at night. It's hot-spring fed so even though the temps are low at night in January, the pool is wonderful. When we were there, I met a teacher from a charter school from Sacramento who took a group of teenagers on a field trip/class. He clued me in on some of the things to see/do there. The geology of the area is fascinating.
Scotty's Castle was cool, we didn't do the technology tour but I'd do it next time. Both of the kids liked it.