Of course, it depends on how much you wish to spend. I know I appreciated the nice David Clark headset I was given as a certification gift. I kept my old headset for passengers.
The person likely took a photo after passing the flight test. Maybe frame that nicely? Perhaps put that together with their 'post solo' photo, and a nice shot of the aircraft they trained in, maybe a shot or two of the airport?
A really nice flight bag or chart case is also an idea.
Membership in an organization such as AOPA or EAA? A subscription to a magazine like Flying or Plane & Pilot or somesuch publication.
Are there any gadgety things the person would like to own, but doesn't? Handheld nav/com or GPS? Electronic flight computer? Very useful, but definitely higher-end gifts.
A spiffy flight jacket? A set of Ray-Ban aviators? A really big watch with a flight computer? Again, more expensive choices.
Knick-knacks and t-shirts and things are fun, but they tend to pile up after a while. If the person is new to flying, he/she may not have reached critical mass on this sort of thing. You can find pretty much everything with airplanes on it. Perhaps a wooden desktop model of the type of airplane he/she trained in or took the test in?
Sporty's is sort of the go-to catalog for pilot stuff. There are loads of similar places online, too. If there is a good pilot shop or FBO nearby, they usually stock gifts along with apparel, gadgets, etc.