If you're anything like me, chances are you have more nice dresses than you really need. Your friends probably do too. Here are some ideas for you.
Go to the Theatre
According to old people, no one dresses up for the theatre anymore. I suppose people are more casual than they might have been, but you still see plenty of people dressed up and you wouldn't look out of place in a nice evening dress. See what's playing in your area, buy the cheapest ticket and dress 'to the nines'. All the old people in the audience will love you for it.
Have an Overdressed Picnic
Get some friends together and wear your favourite dresses (and/or suits) for a very posh picnic. Make sure you have plenty of picnic blankets so no one ends up sitting in the grass and bring cloth napkins to tuck down the front of your dress - picnic food can be messy! Bring some fairy lights or garlands to put in a tree. Everyone else will be jealous of your pretty picnic.
Wrangle tickets to a Uni ball
Universities have balls all the time, but some people manage to make it all the way through their degree without ever attending one. They're usually held by residential colleges and academic societies (law societies always seem to have good ones). So if you know anyone living at college or members of uni societies, pester them to take you to a ball. It's a good night out and usually pretty affordable.
Host a Viennese hour
Have you heard of a Viennese hour? It's basically a really fancy dessert buffet. I've read vague references to an age old tradition at Viennese balls, but I have a sneaking suspicion that this 'tradition' was invented (or at least perpetuated by) the modern wedding industry. Either way, it sounds fun. Tell your friends to have a light dinner and bring over their favourite dessert. This is no Gilmore Girls-style junk food pyjama party - serve everything in pretty dishes, put on some Mozart and dress like a pretty cupcake. Maybe try and teach each other the Viennese waltz.
Eat at a really fancy restaurant (or bar)
Ok, this one isn't exactly a bargain option. Look up the really posh restaurants in your area - 5 star hotels usually have at least one. People get very dressed up for dinner at these kind of places. Make sure you find out in advance how expensive it really is and whether you're able to actually have dinner there. If you can, yay! have fun. If not, go to their bar and sit on a glass of house wine for an hour while you pretend you're waiting for a table.
Extra tip: if you're really worried about looking overdressed, carry a gift bag. People always get a little more dressed up for birthdays - and you can keep snacks in the bag to save money on dinner! Genius.
Have a Cocktail Party
Look up recipes for fun cocktails in advance to work out what you'll need. Assign a bottle of spirits to each person coming and stock up on garnishes and mixers. Take turns making drinks for each other and serve all your food on toothpicks. If that all sounds too hard, hire some mobile bartenders like these guys who use old airline trolleys and dress up as pilots. They sound awesome.
Go to an Orchestra concert
The dress code for orchestral concerts always seems to be slightly higher than any other live event. Most major cities have amazing orchestras and you might be surprised at how much you enjoy it! If you're in Sydney, it's an excuse to go to the Opera House, have a drink at the Opera bar, and pretend that this is what you do all the time. Do it!
A Posh pub crawl
A guy walks into a bar dressed as a rabbit. People think he's a freak. Ten guys walk into a bar dressed as rabbits. People mutter 'Must be some kind of pub crawl...' and go back to their drinks. There's truth to the idea of safety in numbers, and themed pub crawls exemplify that completely. Get a group together, plan out your route and go all out with the formal wear. Just be sensible and agree as a group that there will be no shots and absolutely no singing. Don't be That Group.
Go anywhere after 11pm
I have a theory that after about 11pm, you could go just about anywhere in formal dress and people will assume you've just come from some kind of event. Grab some food at McDonald's then go out to your favourite bar. If anyone asks, say you've just come from a wedding and make up all kinds of scandalous stories about it.
Do a photo shoot
If you really are all dressed up with absolutely nowhere to go, get familiar with your self timer and have a little photo shoot. Get your hair and make up looking lovely and photograph yourself in your garden, or in your favourite room. Give the pictures to your mum for Mother's Day. Mums love that kind of stuff.