Good set of floormats. I like Weathertech, but there are several good brands available.
Running boards or nerf bars if it's 4x4 and doesn't come with them. N-Fab was the brand I went with
Seat covers
Aux fuel tank so you won't be at the mercy of having to pay higher prices in a Podunk town. I'm a month into owning a diesel and I'm more acutely aware of prices now. With gas, there wasn't as big of a price difference of buying along a major highway vs a few blocks into town, but I have noticed with diesel that the HIGHEST places, sometimes up to .30 cents/gallon are along the major roads
Courage is the feeling you have right before you fully understand the situation