I hear you when you say "Sometimes I wish I could run and keep up with my grandchildren but am so thankful I am a big part of their lives"
My littles are very active, and we are involved. Here are some things we can do together without being able to run.
'Venture walks - pick a topic, take a walk, have them find whatever relates to the topic & talk about it - examples, count the squirrel nests in trees, find every different kind of tree leaf or seed and try to ID, count all the different colors of houses or cars you pass and tally them on a little wipe off board.
Nature walks - we have a tiny piece of woods near us where we walk, they climb brush and fallen trees, we examine wildlife signs
Gardening - They each plant their favorite flowers in pots - one pot for Mom, Dad, Auntie & themselves. They have a tiny plot in my yard to plant & tend. Then we challenge them to keep the plants alive... They also help pick up sticks, and the 6 year old is now allowed to help me prune.
Art - messy projects on the patio. Think anything parents may not have time or patience for, and child guided. Painting rocks, water play with food coloring or expanding gel beads, bubbles, sidewalk chalk... I just make sure they have a big old tshirt to replace their clothes and dry things to change into. Or play legos together at the table if you can't get on the floor.
Food - cooking (4 year olds can make scrambled eggs!) food prep - fruit or meat & cheese kabobs, top English muffins, tortillas or flat bread for individual pizzas
What activities do you do with your grands?