Depending on her personality and how well you know her, it could go to the rauchy side for giggles (My Mom is 76 and would love a slightly "naughty" party as she is not "old" in her option) or something they could enjoy such as an assortment of teas, coffees, cocoas, wines etc if they are the leisurely sort of couple.
Electric photo albums are nice too so they'll have a slide show of their pictures from before and after.