I thoroughly approve of this plan, the mandolin is a great instrument to have in sessions where people are mostly playing guitars. It fits in well and adds an extra sparkle.

You can pick out little riffs and counter-melodies - the instrument is better at this than at chords really and in this respect differs from guitar I think. It helps to practice your scales so that you can easily play tunes. Open tunings work well also: GDGD is an obvious one.