If you want something on the practical side, I’d second the nice thermostat or gift card suggestions. I think some frozen food online retailers also offer gift cards; that would be neat so they could choose exactly what size and quantity they need as the snake grows.
If you want something on the fun or whimsical side, maybe a really fun hide, piece of decor, or background for their snake’s enclosure? You’d have to take a look at the size of the enclosure and what they already have in it, and of course make sure it’s snake safe, but I know I always look longingly at the cool naturalistic hides at pet stores before my wallet gets the better of me.
Ooh! Or maybe a nameplate or sign for the tank? That could be on the cheaper side, a really well decorated piece of paper, laminated, with some Velcro tape would be effective. Or go nuts with an actual sign lol.