You might find the perfect art/rainbow party craft or activity here :) 36 Beautiful and Engaging Rainbow Activities for all ages: Babies, Toddlers, Preschoolers and even Older Kids! From Fun at Home with Kids
Scooping items floating in water: this activity can be done with any item that floats, and can be changed around themes, holidays, or to increase difficulty. Children work on motor skills, but can also start incorporating color matching or sorting as time
This simple (and very messy) technique for dyeing Easter eggs uses food dye and shaving cream for a bright, swirly finish. It's great for toddlers because it's so sensory - head over to Simple Home Blessing to find out more.