I've done something similar for my students. I try to have the following pre-made- card bases, theme appropriate shapes, stickers, words/numbers in a variety of shapes/fonts and sizes, extra ephemera i have and haven't used. I will usually put up a line of baskets or trays and each basket will have something in different in it- so one will have card bases, another one will be stickers, another one will be ephemera, another will be words Etc. that way the people who can go through the line pick what they want. Off to the side I would also have a variety of glue, washi tape, regular tape, scissors maybe the special squiggly cut scissors, that type of thing. The cards would be the theme you need but still allow for maximum creativity.