My in-laws just got their new home done. Can anybody suggest any house warming gifts?

You can gift her a terrarium made by you. It will add some greenery to the house and will definitely make her happy :) Here is a link which will help you in making the terrarium -- http://inhabitat.com/diy-how-to-make-your-own-green-terrarium-to-keep-or-give-away-for-the-holidays/