We're going to launch our new, upgraded intranet on the one year anniversary of our company in July. In your case, it seems like a good opportunity to have some kind of contest and then have employees use a poll to vote. The contest could be submitting a favorite office photo from over the years or something. Winner's photo gets featured somewhere and they get a gift certificate.
Or you could crowdsource using Ideas about what people would like to see the company do in the next 25 years!