You may have to water the paint down a bit if it's too gloppy. Turn the sponge upside down and back again to distribute the paint. The trays are used like stamp pads. I usually offer 4-6 colors. Use the basic stamp printing set-up.
Vegetables make nice prints for paper placemats.
Try different fruit for a change. Citrus fruit with citrus colors are cool prints in the summer. Lift up the leaf carefully. There will be a negative print of the leaf. Roll another color paint over it with another roller.