If you'd like to fill it with mostly perennials that they could plant out later, then Rudbeckia 'Goldsturm' would be a start. Anthemis tinctoria - there's 'Sauce Hollandaise' which is cream, or 'E.C. Buxton' which is a liitle deeper creamy-yellow. Various Chrysanthemums could also be chosen - you'd need to check the names and colours in a catalogue or on-line. Dahlias? There are plenty of those around to plant in pots now with the right colours. By now, your crate will be pretty full!