Is Tivoli Gardens open on Easter? I can't seem to find it on the website. And, if it is open, will it be completely packed/more crowded than usual? I'm debating whether to go on Easter or the day before.
Tivoli Gardens opens again on the 31st of March and they are open during easter every year. Since a lot of people have time off during Easter and around, it should be expected that there are more people, but it's rarely too bad (though that is subjective, of course). I wouldn't that discourage me from visiting Tivoli at that time - and I can't imagine that crowds will be that different between Easter Sunday vs. the Saturday before Easter Sunday.
I don’t know about Easter but suggest you may also look into Hadrian’s Villa if it interests you. I love gardens but unless I missed something it didn’t take all that much time to tour the Villa d’Este and the gardens. We took the bus from Rome to Tivoli, the bus to Hadrian’s and back again to Rome. Safe travels
I'll supplement broccolihaar's information with a warning. In Denmark Easter holiday lasts from Maundy Thursday (6 April) to Easter Monday (10 April), so if you want to visit when there are fewer people, you should visit outside these days.
See https://www.tivoli.dk/en/praktisk/aabningstider for opening times.