I got mine a 'double circle' necklace which are quite on trend at the mo that I gave to them on the morning of the wedding; and a Rituals gift set each that were given to them during the speeches to say thanks for everything.
Love the jewellery idea, but agree with Redglitter go for something generic they'd wear again that doesn't focus on the wedding date, maybe their initial on a pendant if you want something personalised? Have a fab wedding!x