Start with planning for different interim and final project celebrations during the project planning activity. That way it doesn't get forgotten and is included in the plan of activities, and the budget is there to do something! Surprisingly, some unique celebrations are not necessarily expensive, especially in the grand scheme of the overall project budget and the financial benefit the company is getting from it.
Doing a good job at celebrations can provide great intangible rewards to the company, not just the individuals. Look at it this way: people often want professional respect, development opportunities, and recognition at least as much and sometimes more so than financial rewards. Teams must make use of all kinds of rewards and recognition to make team members fulfilled and happy on even the most challenging project.
The most meaningful and beneficial celebrations recognize the team for their specific achievements at major points in the project and provide an opportunity for some real fun. True recognition is not just a bland "they met the alpha start milestone," but a personalized acknowledgment of the tangible achievements, the commitment put in, and the odds overcome. Often, even small (but personal) tokens of acknowledgement and appreciation go a long, long way. And the fun aspect—the chance for team members to do something new and not work-related—can re-energize everyone. Another way to think about the fun aspect is the word "hoopla." Every celebration doesn't have to be about a big formal party with recognition speeches. Creating hoopla for the team refers to finding opportunities to celebrate being a team, like doing a fun activity together when a particular milestone is met, getting silly mugs, or whatever would work as a positive in your company's culture.
At the end-of-project celebration, be sure to recognize every member of the team for what they've contributed. No one wants to be excluded. That doesn't mean that specific individuals who made significant contributions shouldn't be recognized specially, just that everyone has made some level of contribution, and your goal is to also recognize and reward the entire team. Also, recognize what the team has done together—particular problems that could not have been solved without cooperation among groups, or a challenging cross-functional milestone that was met because everyone worked tightly together, or aspects of the project's deliverables that just couldn't have been done without extraordinary coordination.
We have captured many different celebration activities from across a range of industries in our Project Team Rewards and Recognition Guideline to help you think of ideas that would work for your project team.