If it’s plastic, so long as you sanitize it first, it should be ok, but I would avoid anything fabric, since the dye might bleed into the water.
My main two things with putting Non Fish Items in a For Fish Space are
Is it likely to mold and/or rust? Yes? Don’t put it in.
Can you safely put it in your mouth/is it food safe? No? Don’t put it in.These are great, simple guidelines 🙂
Also, if it’s for a betta, be very careful of even slightly sharp edges like pointy leaves. Betta fins tear very easily, and any projecting edges will eventually be brushed up against.