J.M.
I would give a check or cash. It's a fairly customary gift. And I would give whatever you can spend (ideally between $50-100) in multiples of 18. In Hebrew, letters and numbers are the same (like Roman numerals) and the numbers 1-8 spell the word "life" so it's a symbolic number. So, for example, I got a lot of $72 checks. (You can make a little note on the bottom that says 18+18+18...)
If you are uncomfortable giving cash, perhaps you want to consider a gift card in a similar amount to a store where there is a wide variety to chose from - I liked the person who suggested amazon. Good luck.