If you know they are into sex toys already:
Pay attention to what they like (colours, shapes, materials, use). If they own a lot of toys and not one of them is a realistic dildo, don't go getting them a porn star replica. If, however, you've noticed they have a fondness for whips, maybe get them a new spanking toy, like a flogger or cane. Listen out for any mentions of new things they'd like to try, or maybe test the waters by slipping questions in ("Steel toys look like so much fun, don't they?", etc).
If you don't know, or know they are new to toys:
Avoid anything too intimidating! For female sex toys, probably stick to small clitoral vibes unless you know they love g-spot stimulation (then get them something small and cheerfully coloured). With Bondage, Lovehoney's own Tease Me range would make lovely first timer gifts, I reckon.
Either way, don't cheap out! I'd rather receive no toy at all, than a cheap £5 vibe as a gift. Gifts should be special. Get something that you know has nice packaging, and if it doesn't, take it out of the nasty stuff and put it in a cute fabric gift bag or something. Gifts should also be fun. I know I don't receive practical gifts terribly well, so probably avoid getting a big bottle of lube or something (unless you're planning on getting a vinyl sheet too and writhing around on it together covered in the stuff ;) ).
It's also probably best to get something that you can use together (unless you're away a lot, or in a long distance relationship, then giving it with a note along the lines of: "think of me when you use this" would be nice).
Also, feel free to push boundaries a bit, but don't go too far. A spreader bar when they enjoy a bit of bondage: good. A full on sex machine when they've shown some mild interest in bullet vibes: bad.