Adventure games, puzzle games. I dunno, I think if it's anything too hardcore then you'll just walk away from it feeling like it wasn't enough.
Personally, I prefer playing the shorter games. I don't mind an long epic game here or there but too many tend to burn me out.
This list is quite full of games that can be ended in under 1 hour. In fact a lot of them need only 5 to 30 mins at most, especially in the SP section of the various genres. I have to find a script that will go through my list to find them all. Many I personally ended but over the time forget the exact name to just point out to ya. A great many of them have a mechanic in which you will die to fade in under 5 mins so the game is just that long. So here are some from the top of my head....
Peggle - World Of Warcraft Edition
Super Vampire Ninja Zero
Broken Cave Robot
RunMan: Race Around The World
Attack Of The 50ft Robot
Light And Darkness
Merry Gear Solid: Secret Santa
Merry Gear Solid 2: Ghost Of Christmas Past
Trilby's The Art Of Theft
1213 - 'Immunity'
1213 - Episode 1
1213 - Episode 2
1213 - Episode 3
Don't Look Back
Resident Evil - Raccoon Madness
SURVIVOR: The Living Dead
Ben Jordan: Paranormal Investigator - Case 1: In Search of the Skunk-Ape
Ben Jordan: Paranormal Investigator - Case 2: The Lost Galleon of the Salton Sea
Ben Jordan: Paranormal Investigator - Case 3: The Sorceress of Smailholm
Ben Jordan: Paranormal Investigator - Case 4: Horror at Number 50
Ben Jordan: Paranormal Investigator - Case 5: Land of the Rising Dead
Ben Jordan: Paranormal Investigator - Case 6: Scourge of the Sea People
Ben Jordan: Paranormal Investigator - Case 7: The Cardinal Sins
Fedora Spade: Case 1 - Prologue
Fedora Spade: Case 2 - The Red Ring
Fedora Spade: Case 3 - Death Wears a Fedora
Fedora Spade: Case 4 - The Last Job
What Linus Brockman Sees When He Closes His Eyes.
The Devil's Tuning Fork
I'm leaving shooters out but there are quite a few of those that are very short but very difficult.