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Nostalgia only hits me when it's something I've not thought about in ages. Lego and vidya are immune because they've remained at the forefront of my hobbies, but the rest of my toys that were part of the great foolish purge of '98, when I transitioned to secondary school, became a 12 year old man, and offloaded my hoard, are likely to set it off. I couldn't point to something specific because it's usually only a temporary rush of gleeful remembrance that won't reoccur in the short term.
Anons dumping scans of Argos catalogues and Toy R Us leaflets have the biggest chance of kicking it off. There's almost always a wave of figures, or a board game that'll spark that "Oh shit! That thing!" feeling for something I owned, or some random toy I picked up at a market/carboot/jumble sale back then and never knew what series it belonged to.