One of the simplest, and most appreciated act of love you can do it to simply
fill her car with gas.
And while you're doing it, why not give it a good wash as well! As with most tips, offer more than once, and give details, like when you'll have it back to her.
She won't realize how much pressure she feels to get gas until she doesn't have to get it.
This is also a great repeat act of love. Tell her to call you every time she needs a tank and you will take care of it.
Gas her up. She'll appreciate it!