In the key of C:
A,C,D,E,
C,D,E,G,
D,E,G,A,
E,G,A,C,
G,A,C,D,C
Use your first four fingers for each set of four notes. Of course you could continue this up the keyboard for another octave or two. Also try playing it in reverse:
D,C,A,G,
C,A,G,E,
etc.