Copenhagen - Day trips Roskilde and Malmo

These are both regional train trips were there is no need for reservation or early bird discounts, so there is no need to buy train tickets in advance. look up schedules at www.rejseplanen.dk
The Malmø leg is run by Øresundstog, so you need to buy tickets from their machines at Copenhagen central.
https://www.oresundstag.se/en/
However I question the choice of Malmø as a day trip, especially since you ask about what to see there in your other post. Helsingør would be much higher on my list of day trips form Copenhagen. It has Kronborg Castle, famous as the setting of Hamlet, an award winning maritime museum and a pretty old town.

And, if the desire for Malmo is for a quick visit to Sweden, taking the ferry to Helsingborg from Helsingor is quick and inexpensive, and there is a nice walk through the central area and up the hill to the park at the old site of Karnan

I was there in December, and did two daytrips: one to Roskilde and one to Helsingor, both wonderful!
Just buy your tickets at the train station; or if you have the Copenhagen Card: they are included on that.
Roskilde is just beautiful, full of history.
The Cathedral is well worth a visit, and the Viking Ship Museum too.
It's a short walk downhill from the Cathedral, through a park.
Helsingor was great too.
The castle is enormous; I went for the Christmas Market there, so didn't tour the actual castle.
There is a wonderful Maritime Museum there too that is one of the best museums I've been to anywhere, so don't miss that.
It's right next to the castle.
I would recommend getting the Copenhagen Card though it's not cheap.
It covers all transport around Copenhagen and the region around.
Plus many, many museums and attractions.
It's just more convenient that carrying around cash for tickets, and you can jump on and off buses when you get weary.
So much history in Denmark, I just loved it!

A word of warning: I found Malmo really rather dull as a day trip. I enjoyed going across The Bridge as seen on TV but was a bit stumped for things to do once I got there. It’s fine for a couple of hours to say you’ve been to Sweden & to grab something to eat and have a wander, but beyond that I was at a loss.

I found that my day trip from Copenhagen up to Helsingor for the castle & amazing maritime museum, stopping at Louisiana art gallery for late opening on the way back, was far more enjoyable.

OK thanks all, looks like Helsingborg will be the winner! Thanks!