Yeah, I've brainstormed about this before, too.
https://worstgen.alwaysdata.net/forum/threads/what-could-scopper-gaban’s-“gift”-to-the-strawhats-be.62885/
Right now, I'm leaning towards Option 10 (Medicine), as the most likely option. Strange diseases seem to be more and more of a plot point in this manga, so I could see Scopper giving the Strawhats some vaccine that they need so they don't catch something like Buggy's "Island Disease" right before they reach Laugh Tale. Maybe Scopper even gives them his own blood, so Chopper can synthesize his antibodies since he's already been on Laugh Tale. Everyone becoming "blood brothers" with a member of Roger's old crew might be pretty fitting, if we're setting off for Lodestar and Laugh Tale right after this.
But, most of these other ideas are still mostly on the table, too. Scopper's gotten pretty banged up this arc, so I don't think Option 7 (Scopper Joining the Crew) is really on the table, anymore. But, everything else is still viable. He COULD teach the Strawhats about Haki Knots. Or give them some way to go underwater or underground. Or finally let them unlock fast travel around the World. Or just tell them more of what he knows about how to fight the God's Knights and how to get around their Regeneration.
It could just be as simple as telling them a story, too. But, like I say in that old post, all the members of Roger's old crew that we've met so far have given the Strawhats a piece of advice AND gave them a physical object, too. Crocus gave them their Log Pose, and Rayleigh gave them the Bubble Coating for their ship. So, chances are, Scopper will give them a physical gift of some kind, too. It's just kind of hard to say what it'll be.