Suggestions for improvisation:
Using the same progression as Howie Day's "Collide", this is fun to improvise on. Chords are (capo 4th fret) |G |Dsus/F# |Emi |C |.
To improvise, try B Major Pentatonic or B Major Scales. If you use the capo while improvising, think G Major Pentatonic (same notes as E minor) in open position.