Halloween parties with scary costumes are now becoming much more popular with each year, and you will see Halloween-themed decorations and even a few pumpkins here and there. But, all in all, this day is still relatively quiet and you shouldn't expect this day to be a big event in the Scandinavian countries.
If you happen to be in Denmark around Halloween, visit Copenhagen's Tivoli amusement park, which celebrates Halloween in Tivoli and Christmas in Tivoli with lots of seasonal activities and festivities, many open rides and additional opening days/hours!
Instead of wide-spread halloween decor and tick-or-treating, there is All Saints' Day after Halloween, a quiet holiday in memory of loved ones that passed away. All Saints' Day is a day of reflection. The long-standing tradition of lighting candles on graves is still widely practised by family members.
Generally, Scandinavian businesses remain open on October 31 (as well as November 1), and the celebrations should have no effect on travel (see Holidays & Festivals in Scandinavia).