Some low-effort, high-impact activities that come to mind are safe sensory play and mess-free art explorations.
For sensory play, a simple go-to is a bin of rice or pasta for your child to explore. You can hide little toys in there for them to discover, or even just let them dig around and create their own stories. For mess-free art, stick some contact paper or clear plastic wrap on a table, with the sticky side up, and provide your child with various colorful construction paper pieces that they can place and peel onto the sticky surface.
Another favorite is the good old shoe box size aquarium. Place an empty, clean shoe box (with the lid removed) in front of your kiddo, and let them decorate the interior with stickers or drawings to create their own mini underwater scene. It keeps and extends the fun for hours!