I wonder what potentialy usable info can i ask windows for later

potentially use in game?

I liked the tricks for example game detected that it is x-mas day

or 1 april and acted differently..

I also wonder if some hardware benchmarking about display haracteristics/cpu/disk?

could be used - and how?

What could be usable or fun to use?

Probably the most useful things are:

- Use their desktop resolution to automatically pick a default resolution to run the game in.

- Use their graphics card info to pick default quality settings.

- Use their speaker configuration to pick the default number of audio channels.

Well, I think information on the location of passwords could be fun to know.

A bit more serious would be to check for example the internet speed for a network game.

I also wonder if some hardware benchmarking about display haracteristics/cpu/disk?

could be used - and how?

There are some games, of which I don't know the names, that ran a benchmark when the game first started. The game would test your graphics and computing speed to set the default settings. It would be easier however to use the GPU and CPU information like Nypuren suggested.

The current year is 2014. Please live in it.

2+ decades ago it was often necessary to either have the user enter configuration values or have the software probe the system, in an attempt to determine which hardware to use. Game developers would buy and use libraries that solved common problems at the time. There were dozens of audio hardware configurations, and if you probed them in the wrong order it could crash certain machines. There were a large number of graphics cards and they quickly converged on the VESA standard, but some cards had better performance with certain algorithms than others, so third-party libraries would run some tests on them. Some cheap cards reported identical information were actually poor knock-offs that crashed or otherwise misbehaved.

Game developers didn't run all those diagnostics for fun. They ran those diagnostics because they were required to give a good game experience.

You also ask about specific dates, Christmas and April Fools Day. Those are easter eggs. Include them if you want, just recognize you are spending development time on a feature that will be seen much less than 1% of the time. Most studios realized they were mostly a waste of time and spent the development effort on real features.

These days we have standard systems that are much higher quality. Since you are on Windows, take advantage of those functions. You don't need to probe to see which audio card is supported, probe to see if you need specific ATA commands. If the interface is available it is acceptable to use.