First of all, start a conversation with co-workers and try to plan things that you want to do for giving a surprise to your boss.
Then check the HR policy for the occasion and include all those things that do not violate any discipline of the workplace.
Arrange a small party at workplace and bring cookies or desserts that your boss likes. However, try to keep things light as it is not a big party.
Also take help from the IT department and fix greeting messages on the computer of your boss. You can also drop a colourful stress ball, a colourful paperweight on his desk that will give a soothing effect.
Make sure you are honouring your boss and do not do it for gaining undue favours from him for your promotion etc.
Try to crack many jokes relating to bosses but make sure they are not annoying. The purpose of doing this should be to start the day with a laugh.
Go to the office or desk of your boss along with all colleagues and give him the day's greetings.
Give him a bouquet along with greeting cards for the day and wish him best of luck.