Tivoli Gardens is a truly magical amusement park in the center of Copenhagen. They are open for Easter/spring, summer, Halloween, and Christmas seasons, and every season features fun and festive decorations and experiences (though note that the park does close for several weeks in between seasons, so check their website to see if they are open when you visit!). You can buy tickets just for "entrance" if rides aren't your thing, but note that rides are an additional cost. If you are visiting with kids, I would suggest avoiding peak times if possible, as it does get very busy, especially on weekends and during school holidays.