Make It A Surprise
Go the surprising way since they might get accustomed making this important event lose value. Do not let the managers make the decisions on who deserves the reward. This might raise suspicion of favoritism killing the morale you will be trying to achieve in the first place.
When you have reached on who deserves rewards, provide a selection of gifts for each contestant employee to choose. Let the supervisors write personalized thank you notes to the winners. Some of the rewards might be hampers, vouchers, chocolates, wine, a day out, certificates or spa days.