For me it was this game called Crysalis on the 8-bit NES. Ended up playing that game for weeks. Then someone stole it from the rental place and I couldn't find it anywhere. Eventually purchased it in another city.
Then on the computer I got into Ultima 7 The Black Gate. I wandered the game world exploring for weeks without even following the main storyline or knowing it existed. Before I knew it the summer was over lol.