On the midsummer night we burn big bonfires usually on the edges of lakes. As you sit on the platform you can see the smoke rising and light of the fire from many sides of the lake. The old tradition comes from the belief that it keeps the evil spirits away: as on special nights like midsummer when sunlight stays put, spirits are thought to be restless.
Midsummer was also a celebration of fertility, which is why still a lot of traditional ways to improve it are part of a midsummer night. Midsummer magic is still used a lot, now more as a game or entertainment, also as memories that will pass on to new generations.
Most common trick is to pick seven different flowers and put them under your pillow for the midsummer night, and your future husband will appear in your dream.