I lost my dad about a year ago so I know where you're at. Some people will look at this thread and say, "Why the hell is he posting here now?" The answer is simple: right now there's really nothing you can do, and something like this you just want to talk to someone - anyone.
When my dad died I had a lot of time to think and remember. I talked about him to my wife and siblings, of the things he used to say or do that would always be with us in some way. It was strange the things that you think of during these times and you discover what about them really made them special to you. It could be something as inconsequential as a conversation or something as momentous as an event or action that had a major effect on your life. It could be the way they talked, the way they smiled, the way they smelled. But something that is very personal to you that made them...them.
Your grandfather sounds like he had a good life. You spent the day with him yesterday. He was a part of your life that nobody can take away and even though you wish you could have him back, remember what he meant to you and what you meant to him.
I'm sorry for your loss, but I'm glad you had so much time with someone you loved who obviously loved you back. And for that you are a lucky, lucky man.