This is a weird one..

But I already accepted a new job about a month ago, but haven't started yet. Another firm has recently shown interest in me, despite me being honest and telling them I already accepted an offer elsewhere. They're trying to sell themselves hard, the HR suggested before I start at my new job, visit them first and get an office walkthrough to learn more about them.

Is there a point? I'm not sure if this is a way to suck me in and throw an impromptu interview or something...

I would seriously consider this place in the future, but the timing just didn't work out. I don't want to burn any bridges but as of now, especially with a signed offer already worked out at another firm, there's pretty much 0% chance I'll change my mind to work here.

Politely decline and tell them you don't want to waste their time because you want to honour your commitment to the current signed offer.

If they insist on bringing you in and they said don't worry about it, go anyways. It's always good to create some connections and make a good impression. You may never know if that will come in handy. Also you seem confident in the current place but you might start and learn that it's not the right place for you.

If I had no ties to the new company aside from accepted offer, I'd entertain the 2nd offer. Sounds like they'd beat the offer.