It just isn't the same without someone on a keyboard, but here
is how I play the guitar part for this song:
Iron Butterfly - In-A-Gadda-Da-Vida
There is the part everyone knows:
E-|---------------------------------------|
B-|---------------------------------------|
G-|-----------------------2--1--0---------|
D-|--0---0---3p2------0------------3------|
A-|---------------3-----------------------|
E-|---------------------------------------|
                                                                               
This is repeated over.. and over.. and over.......
For a little variety, sometimes change the last note
to a C (3rd fret, A string).  Doesn't add much, but
even a little helps.
                                                                               
The little ascending part:
E-|-------------------------------------------------------|
B-|-------------------------------------------------------|
G-|-----------------1--2--1--2--3--2--1--2--1h2p1---------|
D-|--0--2--3--2--3--------------------------------3--2----|
A-|-------------------------------------------------------|
E-|-------------------------------------------------------|
                                                                               
E-|----------------------------------------------------|
B-|----------------------------------------------------|
G-|----------------------------------------------------|
D-|--5--5--5-4-3-2--2--2-3-4-5--5--5-4-3-2--2-3-4-5----|
A-|----------------------------------------------------|
E-|----------------------------------------------------|
                                                                               
E-|-------------------------------------------|
B-|-------------------------------------------|
G-|--6--6--6-5-4-3--3-3-4-5-6--6--6-5-4-3-----|
D-|-------------------------------------------|
A-|-------------------------------------------|
E-|-------------------------------------------|
                                                                               
E-|---------7--6h7p6---------------------|
B-|--7vvv-------------9--7---------------|
G-|--8vvv-------------------9b11vvvrb9---|
D-|--9vvv--------------------------------|
A-|--------------------------------------|
E-|--------------------------------------|
And then just go back into the main part that repeats about
a trillion times.  Pretty simple song.  Two more notes:
For a solo, go heavy on the wah and stay in the D minor pentatonic
scale (hey, it's overused but in this case, you can't go wrong
playing this over that main riff).
And then, periodically freak out through the rest of the song--
see what kind of noises you can get your guitar to produce.