First riff: G D GGG D

Rainy Day, Rainy Rainy Day.

Main riff: (or anything close)

G D.  C D E D.   G D   C D C D

"Rainy Day, Rainy Rainy Day."

play with G major mixolydian for a good while.

find some melody and repeated grooves.

try G9th stuff.

then work together on G11 and G13 stuff

play c d C D Em F Gm C over the top. 

g a G A Bm C Dm G

After a long while it can be bluesy over this for a little while:

G G G C G...  

Go back to G major.. 

Then after a while bring in the II V 1

GGGG  A D G

back to G major..