I feel like the natural next step is a list of games that offer quick Gamerscore and Achievements, for folks that are more concerned with points-per-minute rather than completions.
As such, this list will be composed of games that are decently fun and offer quick & easy Gamerscore in the 450-600G range for a single playthrough or less, making them well suited for rentals or borrowing.
Or Gamerscore competitions
No grinding, farming or boosting needed!
As always, I encourage discussion and recommendations, and if you write a few sentences about a game you want to see on here, shoot it to me and I'll edit it in and give you credit. Happy waffling!
Fight Night Round 4 - First, download the free update that lets you map punches to face buttons. I recommend L & R jabs and hooks, and use the Right Analog for uppercuts and haymakers. Now, create a custom boxer any way you want except don't put him in the Heavyweight class. Import your created boxer and play through the Legacy mode on Amateur and pay attention to your calendar--use every training session you can and just auto-train. Never sim a fight, and reload without saving if you lose. Focus on maintaining an offensive rhythm in fights. If the opponent clams up and starts blocking everything, then stop and wait for him to throw a punch. Block it and, right when the camera zooms in, counter it. After beating the champ in your weight class, you will have the option to move up to the next weight class. Do this and fight your way to the top again, then retire as the Greatest Of All Time. This will do it for the Legacy Achievements, now just do some Fight Now matches and get the misc ones like 500 jabs. Make sure to knockout a CPU in Round 1. 535G, about 6-7 hours.
Wet - Play through Story mode on Hard. You have unlimited slow-mo so use it! Upgrade Pistol damage ASAP, then buy a health upgrade, then don't buy any other upgrades until you've bought all of Rubi's abilities. Find and grab all the monkeys in Chapter 1, there's only 5. After you finish the story, play the second Chapter again in Points Count mode and beat the target score. 490G, about 5-6 hours.
NFL Tour - Always use Standard rules, don't change anything, and keep difficulty on Veteran. First up: play Smash & Dash. Choose someone fast with good tackle-breaking. Save your Turbo for when the opponent is getting within tackle range. Don't worry too much about the bonus spotlights, just run big circles and keep distance between you and the other guy. Get a win with over 50% possession. Next is Redzone Rush. Make sure you score every time you have the ball--do this by initiating contact with the defender before he hits you, and focus on the icons to break the tackle. If you don't get 5 TD's in a row, exit and restart. Stop at least 1 of the opponent's runs and you're done here. Now for the last mode, do a Play Now game. Find a long pass play you like and use it 4 times in a row. When you're about to score a TD, run diagonally towards the wall and hit the B button to do a wall-run over the line. When the defense tries to blitz you, hold on to the ball and run backwards. The linemen will come at you one at a time; reverse tackles 3 times before getting rid of the ball. Next, go for a big hail mary pass by running way backwards before you throw to someone open, and don't use Turbo at all. Once you catch it, run in for a TD still without using any Turbo. This will pop your 8th Achievement. 450G, about 2 hours.
Madden NFL 08 - You can get 90% of the Achievements in this game by simming. Just go to Play Now, find a team you like with a high score, and play against a team with a low score. Statistically, the NE Patriots are the highest and the Texans are the lowest. For the first few games, play with 5 minute quarters, difficulty doesn't matter much. When choosing your first play, hit Start and go to Supersim. Choose the option to sim until end of game. After the game runs its course, the Achievements will pop when you close the sim menu and go to exit the game. After about 10 games done like this, switch to 15 minute quarters and sim some more. It's up to you when you want to stop doing this, but it will be fairly obvious when you've gotten most of the Achievements. Once you've simmed 3 15-minute-quarter games in a row without popping a single Achievement, it's time to throw in the towel. At this point you can either go back to simming 5-minute-quarter games to get a few more Gs, or play a long game manually to get the 'cheevs for long passes and the like. 525G, about 2-3 hours.
Open Season - This is actually pretty easy to complete and get 1000Gs, but if you're paying attention to this list then that means you want quick points without grinding, right?! Just play a little over halfway through the story, grabbing all badges and getting maximum Wild Points for each level, both of which can be checked in the pause menu. Then spend those points to buy all the Wild Skills. 435G, about 2 hours.
Kung Fu Panda - Play through the entire story on Master Ninja (medium) difficulty setting. Grab all the collectible tidbits in each level, isn't hard at all since none are hidden much. Break everything you can! 460G, about 4 hours.
Star Trek: Legacy - Set the game to Ensign in the options, then play all the way through the story. Don't lose any of your main ships during the game; if one of them is taking heavy damage, warp it to an empty edge of the map and repair everything. Many of the missions have a misc. Ahievement attached to them, just check your Achievement List for the game as you get to each new mission. Most of them are easy on Ensign level. 500G, about 5 hours.