You're kidding! You can't think of things that you did at your office that warrant attention?! I would hope that you know better than your boss what you do on a daily basis, and what contributions you make. You asked your boss to write you a letter, so you're comfortable that his more limited knowledge of your potential/acheievements would suffice. You should be even more comfortable with what you YOURSELF would write! Make sure you use strong language: 'Seattlegirl consistently demonstrates her exceptional motivation and attention to detail in her position. Whenever she is asked to ______ she asks how high...' This is a gift. Take advantage of it!!
I recently got some advice on this front from and advisor who's been doing this stuff for years and has read hundreds (thousands?) of LORs. She said that action must always be first and foremost. A good LOR is more verbs than adjectives. It's easy to say "Seattlegirl has a superb intellect and a caring soul" but hard evidence to back that up is critical to set you apart from the crowd.If you have some writing he's done, try to mimic the voice, or at the very least, try to shake up your own writing voice for this one. A few commas where you wouldn't ordinarily put them (or an extra parenthetical or two) can't hurt.