For people who care a lot about each other, practically every little thing becomes a reason to celebrate. That being said, celebrations need not be grand and fancy every time. After all, there are instances when your ability to celebrate - financial or otherwise - may not be as much as your willingness to celebrate, and there is a need to figure out a way to mark a special someone's special day, even if the purse strings need to be tightened a bit.
Cook (or prepare) his favorite meal. A celebration need not mean a grand party where anyone and everyone is invited; that is reserved for when you have extra cash lying around and are willing to splurge. Sometimes, an intimate meal for two is even more special, and the food need not even be gourmet. You may want to take a quick trip down memory lane and prepare what you had on your first date some X number of years ago - which may only consist of burgers, fries, soda, and Cracker Jacks - and as you snuggle up to watch the same movie you watched so long ago, you show him that Cracker Jack prize that you have treasured all these years.
Take his "baby" out for a little love. Let's face it, most men regard their cars as precious darlings, although they could not help the fact that there are times when their busy schedules simply could not accommodate their being able to personally give their cars the kind of lovin' that it needs. You may offer to take his car for detailing, or choose to do it on the sly and surprise him with a ride that is almost good as new. This is assuming, of course, that your significant other actually leaves the car keys lying around, and that he is not one of those weird types who do not allow anyone else to touch their car.
Get him a gift that's just for him. If you would rather give him something he could use or wear for quite some time, then you may want to consider getting a gift that could be personalized, such as pewter tankards or a set of engravable men's cufflinks. These items may be engraved with such traditional markings as their initials or a special date, or you may opt to be a little creative and choose to write a personal message or saying. While some of the items that you may get for $50 or less may seem generic, having men's cufflinks engraved will transform it into something unique and special.
Let his way be done. For just this one day, let him rule. Whether we like it or not, there are things that couples usually do not do together, because that "thing" is something that one or the other would rather not do. A classic example would have to be camping; most men are exhilarated by "roughing it" and spending time in the great outdoors, while women are repulsed by the prospect of not being able to take a shower or lie down in a real bed. But for this one time, indulge and accompany him on a day camping trip - although you may put your foot down about spending the night in a tent.