Nuts Bait the animals. Unsalted nuts work well and are relatively inexpensive. If you’re in a place where the squirrels are particularly timid, visit it a few days in a row, preferably at the same time. That way they’ll associate you with food and come closer.
Wood props Logs and other bits of wood make a natural-looking portrait when the squirrels jump and perch atop them. Set them up in front of you. Try to do this at your lens’ minimum focusing distance (or a bit further away) to make the squirrels large within the frame.