As someone who has been on the employee side of this situation, I feel I can offer some insight.
To me, the biggest thing is personal acknowledgement - having management actually stop by, in person, to offer thanks for your efforts and let you know the extra work is appreciated can speak volumes. That they took the time - even only a few minutes - to actually come by and express thanks can be a big deal to the employee.
This was actually one of my most immediate complaints I raised during the exit interview when leaving my previous employer. Near the end of my time there, working OT in an effort to bring a project back on schedule was met with zero acknowledgement from management. Having the project manager stop by on his way out the door in the evening to let us know he appreciated the extra mile we were going would have made a big impression, in a positive way.
Now, other ways of expressing thanks can be good, but are going to vary a lot person-to-person. This type of personal thanks seems to be a fairly universally appreciated gesture in my experience.
For an example of how other measures can vary, I would caution generally against the suggestion of public acknowledgement by means of publishing in a newsletter, in a company meeting, etc. While it might seem like a great idea, not everyone appreciates being singled out for that type of acknowledgement. Depending on the individual, this may not been seen as a welcome gesture. Someone who is very introverted will likely not appreciate the spotlight being drawn to them, especially if they also feel that what they are doing is not extraordinary.
The same goes for offering certain types of compensation. This can backfire in a number of ways. Significant monetary bonuses can be appreciated, but also may lead to people purposely trying to work OT in order to "milk it" for extra cash. Some may even see the offer as insulting, especially depending on the amount. For yet others, it may present a very uncomfortable situation - feeling that the offer is unnecessary, while not knowing how to decline or otherwise respond. So, for the same offer, there could be at least four different types of responses, depending on the person, with only one being purely positive.
Again, those of us commenting from afar here aren't really in a position to say definitively which other expressions of thanks should be made in your case. Suggestions can be made of some possible actions, but actually choosing which applies best can't be done without personal knowledge. For some people, things like extra time off or a monetary reward would be great ways to express your thanks; for others, not so much. (And they may actually be received negatively.)
You are in the position to judge what kind of person this employee is and how they may respond to certain responses. Beyond personal thanks, which is pretty much universally well-received, it really should be handled on a case-by-case basis.