The invitation still means you have to be elegant, but you can put some fun into it by wearing a brighter colour in the shirt or tie. Try out that jacket you've never had the occasion to wear before. Some men have the knack of putting an outfit together without it looking contrived.
For a midsummer outdoor party, try a light coloured suit, a blue shirt and a navy tie with a discreet pattern. In many cases, black or white suits are still acceptable, or maybe a wonderful wool tweed suit.
Calvin Klein once took a smalll liberty with black tie by substituting a scarf for his shirt and tie. This might be a bit extreme for most men though.
A lot of men feel hate black tie occasions because they feel uncomfortable in formal clothes. There's more than one way to feel comfortable, and with creative or festive dress, you can feel relaxed. When asked to wear creative dress, just be yourself. Wear what you would normally wear but with a twist. Don't try a new style that would make you feel uncomfortable. Even if you know there are going to be a lot of punk types at the party, don't consider dressing as a punk if you don't normally dress that way. Stay with your image or you'll be constantly worrying if your pins are straight.
If you use your imagination, you can come up with quite a few options:
If you're really unsure what to wear, stick with a nice suit, perhaps navy. It's dressy but not too formal.
What you wear depends on who the invitation came from. Some people mean it to be good fun, while others may mean it be nearly totally black tie.
Don't make the mistake of overdoing it. Don't put too much thought into it. Don't
try too hard.