First up: the classic. Cracking open a bottle of champagne (or cava/prosecco if you're on a tighter budget) is a traditional way to celebrate great moments in life. Something about hearing that 'pop' and seeing the torrent of fizz just lightens the spirits, doesn't it? However, don't exclude non-drinkers: provide a non-alcoholic version or simply a range of soft drinks to make sure no one goes thirsty.
Away days don't have to be expensive. We live in an age of endless deals and vouchers. Sign up to Groupon, Kelkoo or LivingSocial to make sure that when great bargains on days out come along, you snap them up. Treat your team to a day at the races, a trip to the cinema, a paintballing experience or even a walking tour (the Jack the Ripper tour in London is a blinder). Away days not only reward staff for their hard work, they are also great for team bonding. People who have little contact during the working day can find common ground and shared interests.
If you can't afford to take a whole day out of the office, why not bring the treats to you? Hire a professional massage therapist for the day to treat your staff to 15-30 minute sessions at their desks and ease away aches and pains. If you really want to push the boat out, you could have a range of therapists practising everything from reflexology to pedicures!
In China and Thailand, birthdays, weddings and other celebrations are marked by sending lanterns into the sky. This is best done after dark for full effect. Give every member of your team a lantern (keep spares handy, the lighting process can be tricky) and watch as the twinkling lights float up into the sky. Don't forget to make a wish! It's a cheaper (and safer) alternative to fireworks, with just as many 'ooh's and 'ahh's.
You could wrack your brains to think of the most innovative, unique and interesting way to reward your staff for a job well done but sometimes the best way to say 'thank you' is by getting back to basics: a bonus or extra holiday. By giving bonuses to your staff - even small amounts - you let them decide how best to reward themselves. Perhaps they would rather go out to dinner with a friend they've neglected through long nights spent at the office, or buy some new music to enjoy in their spare time? As for holiday, duvet days are all the rage in staff-focussed firms these days. Grant an extra day to high achievers and watch morale soar.