The obvious downside to giving gifts when deadlines are met, is it can become expected rather than a source of motivation. The counterpoint to that is the additional benefit of a well received retention strategy. You most likely already give an annual gift or bonus that has fallen into the expected category and is no longer special to employees. When you have a bonus in place, choosing to raise the bar becomes a difficult decision. The only time it is a good option is if the chosen reward is something you and your employees will enjoy.
It is best to tie the reward to a milestone. Often in construction an incentive is built into the contract with a clause that additional money will be earned for a project delivered a specified number of days early. Your reward system can work essentially the same without the technicality of a contract and a huge dollar amount. For instance, if your client buys off on the first phase of the marketing plan, reward the lead staff person with something that ties to the project. If it is for a golf course, buy them a club. If it is a restaurant give them a gift certificate. If it is a chocolate store, treat them to a box of chocolates. You are only limited by your creativity.
In the case of your vendors and contractors, they most likely are the ones offering you special treatment. As part of their marketing package, they have taken you to lunch or dinner, showered you with holiday gifts, done work off the clock for you, or even sent you on a paid vacation. Consider turning the table on them and take them to lunch or give them a small token of appreciation. If you are a construction developer, a trade contractor appreciation day is a easy way to give everyone a break and not affect your project schedule too significantly. Perhaps a golf tournament after the work day is over or a spa treatment at the clubhouse can be arranged. You will see benefits from indulging your trade contractors to their own special day.
Be sure your rewards are appreciated and most importantly, never allow them to be an expected part of working for you. If that happens, the reward is no longer viewed as a motivational tool. You walk a fine line with gift giving, so be sure you are up for the challenge. Take note of what works and what doesn't, and don't be afraid to try something new. Use your downtime wisely and work to keep everyone motivated.