A starter library of arranged popular and traditional songs, auxiliary instruments (tambourines, triangles, finger cymbals, claves, etc), a nice variety of drums (I love SoundShapes; they’re cheap, easy to store and are perfect for lessons about physics and sound (larger instrument = lower sound etc). These you can add to songs and use as hooks to get their interest and teach a variety of concepts. Also, you might get a few sets of classroom posters. Music Is Elementary has a bunch of nice poster sets for about $10 each. I got a set and laminated them; been using for years.
I agree about risers.