Barnes and Noble at LSL here has a children's department. You might want to go there before he arrives and spend some time choosing two or three picture books with simple, colorful illustrations.
As you no doubt already know, he is not too young for books. You said he is observant which I bet means he already has enough of a concentration span to spend time sitting on your lap, looking at books and "discussing" them with you.
-- You know those books with pictures of animals that you can talk about. He might like to point to the animals and say their "names" and make the animals' sounds. -- He might like books with pictures of big trucks, especially firetrucks, and airplanes and trains, and big construction equipment, too.
I think it is fun to sing with little ones. You might find a song book at B&N with those nursery rhymes we sang long ago or maybe some other songs that little kids love.........
"Down in the meadow in an itty, bitty pool, swam three little fishies and a mama fishie, too"........Anyway.........
There are wonderful memories and learning times to be created with just a few books and a nice comfortable chair for just the two of you.
No matter what you decide to do with the time you get to spend with him, I know you are going to have fun and will love every minute of his visit.
Old School Boomer