Generally I prefer diegetic merch, things that the characters themselves could conceivably own in the story. Books, cups, magical trinkets, that sort of thing.
I see those edge-lit acrylic “hologram sculptures” being sold on alibay and such, they always looked disappointing since they tend to misrepresent the 2D acrylic as having some 3D holographic depth. But something 2D like a magic circle like Siobhan glued to her hand could look really neat with this technique. Especially if the light pulsates. Pic related:
I’m not sure what laser cut (acrylic?) coasters would be like, maybe they’d be fine with some little adhesive feet, but I think they’d need to be at least 6mm thick for the strength, and the laser cut edges look a bit funny at thicker depths. If wood, they’d need to be sealed with something like polyurethane. Laser engraved anodised aluminium is also a material option, it can be thin enough to be used for sturdy bookmarks (if you deburr them properly), the same technique is used to make aluminium business cards, though you only get two colours (and cutouts). You can get a surprising palette of colour from fibre lasers microscopically heating the surface of non-anodised metal like stainless or titanium.
Even ordering printed circuit boards to be coasters is an option, so long as you pick a lead-free finish, the edges are usually free of burrs but may need a bit of sanding to feel really smooth. You can get five different colours on a PCB (silver/gold plating, coloured solder-mask over copper, the same coloured solder-mask over the substrate, white/black silkscreen, and raw substrate) if you want to make (posterised) artwork on a circuit board. Pic related:
Not sure how feasible it is, but DnD dice, dice bags, dice jars, or even dice rolling structures might have enough of a market.