As I wrote in reply to another comment, the present simple is used to describe habits, or recurring actions. Example 2 is a little strange because "I stay in the office" implies a habit, that is, I do this everyday, for example. However, "Today" implies that it's not a habit. Together, the sentence doesn't sound correct.
Thanks for all your replies. This simple/progressive thing is a big problem for me because aspects do not work in the same way in my language.
I generally use the progressive in contexts like this one but sometimes I might end up speaking like Tarzan, or TA(r)SAN.