Any noisy toy that doesn't have an off button or volume button. Fortunately as the kids get older that's become less common.
Any poxy science kits or anything else that requires an insane amount of parental supervision.
Slime, I learnt early on not to buy it myself but last summer ds1 was given it in a party bag, he accidentally lobbed it over the fence into next door's garden. They have dogs so I went over to try and retrieve it as idk if it's toxic. Spent about 10 minutes scraping the shitty stuff off their patio. A real low point.
My ex text me last Christmas asking if I thought a digital keyboard was a good present for ds2, yes I said as long as you keep it at your flat as no room here. Sure enough it did not stay at the flat. It lives in my mum's house.
Ex also bought a fucking huge hardback book for ds1 about musicians, ds1 not the slightest bit interested in music, but ex is so of course I have to find somewhere to put it 🙄
Scalectrix or hot wheels crap or anything else that the kids are excited to put together but that breaks easily, there is only one way to play with it so they get bored within 1 minute but it still takes up space.
I sound like a snob but anything that's shit quality, if you only have £5 to spend that's fine but spend it on a colouring book not something shit that will break within 2 seconds.
Anything that has an obscure battery because no I don't have random unusual batteries in my house on Christmas Day.
One year the in laws bought a hideous electronic ride on Range Rover which is nearly as big as an actual car. It lived at their house for awhile before mysteriously appearing here. The battery died and of course my ex thinks I should replace it.
And yet we feel compelled to keep these things because they were gifts. If I sound angry it's because I am 🤣