Just a note to people who feel they can't contribute much: Be a Bard! There's three great skills here.
Battlefield Overture gives a buff to attack power and critical rate - the attack buff is based on percentage. Someone carrying a massive dual-handed blade will get a large increase. Giants rejoice! Seems to apply to bows as well, but I somehow doubt it affects magic.
Enduring Melody increases defense slightly, from 1 to 11 from ranks N to 1, as well as some modest protection, and perhaps most importantly it buffs your MP and SP recovery rate. Should prove very useful for rafting and ballooning, as MP can become a bottleneck.
Lullaby is an odd one, but works surprisingly well. Maxed out, it will keep an enemy asleep for 9 seconds, which is fairly massive, and provide bonus damage dealt to them. The bard does NOT gain aggro, making it very possible for the bard to sit in the corner and play, providing a stagger effect on enemies in a dungeon room and shifting the multi-aggro threat down a notch. Unfortunately it's based on your CP, and most low-level players will have poor CP and thus would be poor lullaby players.
Then there's the Harvest Song as well, which anyone who ever produces anything will enjoy.
All of these are buffed by your Playing Instrument level, and can receive an "excellent performance" buff of +10% or "inspired performance" of +30%. Even someone low-ranked with enough free time could get their Playing Instrument higher just by turning on "repeat performance" option while standing in a tent. Additionally, Musical Knowledge has a quest line where it gives back most of the AP you invested in it, giving you fairly massive Int boosts for very low AP costs.