From what I remember, a vow renewal isn't a legal thing anywhere. So actually there is legally nothing you must say.
I wouldn't suggest gathering inspiration from The Princess Bride, no matter how cool it is. The only real advice I can offer is bring up their past together. If you know (or can find out) how they met, how long they dated, any funny stories (especially during the original wedding ceremony), and so on. Were they ever apart due to work or war? What quirks do they still deal with? (For instance i knew a couple where the husband said the only thing about her that bothered him was that she never kept her car clean, and that he gave up and figured he'd do it.)
You can even tie life events with world events. Did they get married in the same year as a major event?
If I think of anything else, I'll post!