The German Wikipedia article on diminutive features a section on “Verselbständigte Diminutive”. These words all have conventional meaning, i.e. they are listed in a dictionary by themselves and not as part of their stem entry. Some of the stems are uncommon (today), e.g. Mär and Maid/Magd hence the derivation is intransparent for many people. Many are compounds that denote plants and animals where neither the left nor the right-hand part would denote one and only few could also be used without -chen, e.g. Meerschwein, Seepferd, Silberfisch are not unheard of. One can use the semantic opaqueness to construct bad puns, e.g. “Die neue Frau meines Vaters ist recht klein, ein richtiges Stiefmütterchen.”
I’m interested in the last category, words that look like diminutives but aren’t. Eichhörnchen ‘squirrel’ doesn’t derive from Horn and Plätzchen ‘cookie’ doesn’t derive from Platz, although words like Ruheplätzchen do. (I think Veilchen may be misplaced there, because Veil is not a German word.) Are there any more like these?
There probably needs to be a consonant preceding -chen, since some (orthographic) candidates are nowhere near being thought of as a diminutive, e.g. nouns Knochen, Zeichen, Rachen, Drachen, Rochen, Sachen and several verbs used as nouns like Kochen, Suchen, Erwachen, Lachen, Sprechen, Tauchen, Stechen and, of course, Catchen.