I like me some ap bard.
is my main goal in terms of items then whatever ap fits the situation.
is always good for the ms,
if they got big/magic resist, and so on. If you wanna be boring just go with the generic supporty items you would with soraka or sona types.
collect chimes when appropriate and don't leave the adc without giving him a shrine to heal/escape if you are gone. Once you have a good number of meeps running about you can auto a minion wave or frontline champ and hit the backline with good damage and a pretty gnarly slow.
His q can stun champs to a wall or from other champs/minions so get creative about hitting minions or champs. if you have your stun up and people chase you through magical journey you can boop em with the stun as soon as they leave the tunnel.
if your jung has a weak early jung you can start with his heal and leave a shrine or two to keep him healthy longer, just make sure the adc knows to play chill until you get lvl 2.
the ult is gonna get lots of love/hate. use it to catch runners, stop chasers, freeze a turret to dive (be careful since it'll start back up after the ult stops and the wrong person may get the aggro), when defending your turrets you can either freeze your turret or freeze the enemy minions so the turret shoots the champs (not that turrets can kill people anymore...), or pause a fight until you can get there to help out. Just be careful and remember the distance increases the time it takes to get there. It can work, but be careful freezing the enemy back line. sure it stops the damage, but your team is gonna blow spells on the tanks/bruisers and the carries are gonna come out of the ult squeaky clean.
on big pushes I try to have a shrine or two a little ways behind the team in case we have to bail or if someone just needs some quick health.
remember that you are still the support and need to ward and such, and if you go with my sort of build the ms boots, items, and the slow from scepter are to keep people slow and you fast. you are very frail and should dart around fling your meeps then back off and assess the situation in big slug fests.