Easter eggs or "eggs"
Whether or not you want to spring for the eggs ($$), Easter-egg—or Easter–egg-style—decorating projects are great, because they can be adapted from the ultra-simple and non-messy for early preschoolers, through to elaborate crafts for teens and adults. Even vegans can get in on the fun!
For preschoolers. Unless you don’t mind a lot of broken eggs, waiting for egg decorating until after toddlerhood is probably in everyone’s best interest. Preschoolers are typically ready, though. If your grandchild is patient enough to wait while the eggs soak in the dye, store-bought dyes like the ubiquitous Paas brand, or dye kits that contain extras like stickers or glitter, are lots of fun and low labor on the adult side. Just follow package instructions, add a bit of creativity, and voilá. Pro tip: Because the eggs need to sit while they dye and again as they dry, be sure to have something else in mind to occupy the little ones in the lulls.

For the young ones who aren’t very patient—or for ones who are but just like the bright colors—I can recommend the Eggmazing machine. (No, I’m not getting any kick-backs.) It’s super-simple, non-messy (a big advantage), and results are worthy of centerpieces as well as egg hunts.
For preschool through grade school. Older kids can make all the preschool options more interesting by adding their own flair to the projects. They can add designs to the eggs with clear or colored crayons, get fancier with the glitter decorations, make two- or three-colored eggs with strategic dipping and drying, craft their own baskets, and so forth.

Older kids might also enjoy adding a little science to the project by making their own dyes from food. https://www.designmom.com/easy-natural-dye-for-easter-eggs/ It’s a fair amount of work/time to prepare the dyes, but the natural colors are quite pretty, and you can eat most of the veggies involved, too.
For late grade school through adult. Again, older kids can use their imaginations to elaborate on any of the preceding techniques. But if they want an egg-decorating challenge, dying Ukrainian-style Easter eggs (pysanky; see photo at top) takes the process to an art. The technique is like batik, in that you dye the cloth (batik) or egg (pysanky) in progressively darker colors, and preserve what you want of the lighter colors by covering the parts you want preserved with wax. Then you remove the wax at the end to reveal the bright colors. I’m not Ukrainian, but grew up near a Ukrainian Orthodox Church, where we were introduced to the technique at their annual egg sale. We learned the technique using old-fashioned kistkas. Kistkas are small metal funnels that you fill with beeswax. To create designs on the eggs, you heat the old-fashioned kistka over a flame to melt the wax, then draw on the egg with the melted wax—fun, but very tricky. Modern kistkas are electric, which is more controllable—which is helpful for the fine lines and detail that mark the pysanky style. This video gives the general idea, and this link gives one source for supplies (note the book A Kid’s Guide to Decorating Ukrainian Easter Eggs) but you’ll want to seek out more detail and advice before you launch into this form. That and be ready to discuss geopolitics while crafting.

Vegan alternatives: If you or your grandkids would rather not use eggs, one alternative is to use paper eggs. Obviously, wet dyes are not going to work on these. But markers, glitter, stickers, crayons, paint, sequins, tissue paper, fabric, etc., are all possible decorating materials. Plus there is the fun of figuring out what to put inside them. Candy, of course, or maybe fuzz-and-wire chicks? Or??
Finally, our grandkids have had great fun with making and hiding simple paper Easter eggs. A grown-up can help the kids draw eggs in varying sizes on a sheet of white paper or light-colored construction paper. The kids can then decorate the eggs with markers. After they finish, cut the eggs out, roll up tape tubes from masking tape (sticky side out), and let the kids tape the eggs in mysterious places for hunts for each other or for you. Note: You may be finding hidden eggs for years—sweet reminders of the fun you had!