Maybe doing all the VR missions in MGS2 and then the week after, getting every dog tag over a weekend to get every achievement. Plus all the Kerotan and hunting every animal in MGS3. I was pleased to do this in the HD collection, because when I was younger, I felt like I lacked the patience and skill to ever really try it.
Then, there is going back and finishing Max Payne 3 on every difficulty over yet, another weekend. That was fun. Such an underated game, I feel.