Hey Alex,
Still not sure if you gave your age yet? I grew up in a basement space where I had my second layout on a ping pong table. That was HO. Not sure what gauge you are planning on. If O gauge, not sure the ping pong table would be sturdy enough (at least the one I had).
Lots of good advice and some funny stuff like what El Classico said which is exactly what I was thinking, too.
If you nicely explain that maybe it is something you can do to be closer with your grandfather (or any other interested relatives) and learn some history, about electricity, some carpentry, etc.
Good luck,
Tom