I haven't done the drive in a long time, I have to echo the Andersen's windmill, hundreds of cows at Harris Ranch, and add that we tend to see a crop duster plane just about every time we make a long I-5 drive. Cattle egrets (those tall white crane-like birds) are usually very close to I-5 where the rice paddies and other fields of crops are. If you pass by the Tower Bridge and the previously mentioned "UFO" definitely add those, my kids love the UFO and when my oldest was 2 and we lived in midtown Sacramento we would detour to drive by it all the time!
Since there are a lot of you offering great answers, could anyone recommend a place to stop and eat along I-5 (for breakfast specifically). We're driving down in May, we live about 90 minutes north of Sacramento so I'm figuring anything south of Stockton all the way to the grapevine area because I'm not sure how early we'll be leaving yet. I figure if DH and I are too excited to sleep, we'll hit the road as early as 3 a.m. haha.