Why only 20 hours a day to do whatever you want? Sleep is for wimps.
Ok, seeing as you have enough free time, I would highly recommend a free RPG called, "Nehrim". It's a total world conversion of Oblivion, and when I say total conversion, I mean that it converts absolutely everything. All the annoying stuff that was in Oblivion is gone. The geography, landscape, world, law, character, quests, world, etc, in "Nehrim" is all brand new. Its by far the most ambitious, impressive and well executed MOD I have ever played.
If you ignore the world and everything in it, you could probably blast through the main quest in about 50 hours. Assuming you are curious enough to wander about a bit and try some side quests, then you are looking at 150+ hours of content and gameplay.
As it's a MOD its free, but you do need a copy of Oblivion to make it work. The US version of PCG voted it MOD of the year in 2010, it has won loads of other awards, and is an absolute treat if you like open world RPG.
Unfortunately it does have one slight drawback, which is that it starts inside a very dark mine, and the darkness and frustration in escaping put a lot of new players off. It's by far the weakest part of the game, but all you have to do is brighten your screen if you can't see enough, or read a walkthrough if completely stuck, but once you do escape the mine, the game world opens up, and you have the freedom to go anywhere and do anything, providing you can fight / stealth your way through. In fact you will probably stick with the main plot for a bit, as the world is so huge and so hostile, it helps to establish a base and level up a bit, before becoming to adventurous.
The MOD is a remarkable achievement, and well up to professional standards, and it's the game that many of us who loved Morrowind wished Oblivion had been. The game does not therefore hold your hand, but will instead often force you to think. There are usually multiple solutions to any given issue, as the game has layers, upon layers of depth to it, which you can manipulate to your advantage, depending upon what choices you make.
It is therefore a challenging game, so it will jar badly if you expect it to be as easy as Oblivion. It's not. It has far more in common with games like Morrowind / Gothic / Risen, so even the wild animals will cheerfully rip your head off if you don't treat them with a bit of respect. Likewise with the NPC, as you will have to learn how to combat different foes and with what. It's not simply a case of waving a sword at bad things, and expecting them to eventually fall over. If you engage your brain, then it's an absolute delight to pick your way through the game, but save often, as this is not the cozy/safe world of Oblivion, and unexpected or violent things will frequently happen to your character as you travel through this fascinating world.
Last but not least the main plot is certainly very different to the usual RPG tripe, as it ponders on an age old philosophical question. The game was made by Germans, so they picked a quote from Beethoven to kick it off, but all cultures of all nations, have explored the central theme. I won't spoil it for you as the main plot and your choices in it will twist and turn its way through an exploration of that theme.
Thinking about it, you are going to need the walkthrough at some point plus more info if intrigued / interested, so go here:
If you want to download the game, go here:
Regards - Mr P