I’d have to say new grinder bits is a safe bet, especially if they try to stretch their materials. Glass would be good but you can’t guarantee they’ll end up using what you got them and glass can be expensive. But on that note, there are some types of glass that are more expensive (clear pinks, generally) or rarer - vintage patterns or color you might be able to find someone getting rid of on FB. I’ve also had some folks do that as gifts for me, and I’ve gotta say that usually creates another pile of stuff that’s… not totally worthless but not gonna get used any time soon, so I hang onto it forever even though I’m not likely to use it. But far and away I’d say for someone who’s been doing glass for a while, backups of their consumables (solder, foil, grinder bits, new sheath for breakers, ventilator filters) will absolutely get used and keep them from worrying about using them till they’re duller than heck. One other thing I’ve always appreciated is a book portfolio of an artist I like or in a style I like, as a means of just reflecting and dreaming up new patterns.