This is very, very cool. Speaking as someone who also wrote QBASIC games as a child, this evokes sharp pangs of nostalgia.

I happen to have an old QBASIC book (pub 1993) next to me, and was skimming through it shortly before coming across this article.

Random file I/O :

    OPEN filename$ FOR RANDOM AS [#]filenumber LEN=recordlength

The book then goes on to provide helpful ways to guesstimate at the record length of a file on disk.

Drawing 2-D "staircases" diagonally in both directions across the screen :

    PSET (0,0) 
    FOR p = 1 TO 25
        LINE -STEP(25,0)
        LINE -STEP(0,15)
    NEXT p
    PSET (639, 0)
    FOR p = 1 TO 25
        LINE -STEP(-25,0)
        LINE -STEP(0,15)
    NEXT p
    END

Nostalgia, indeed :P