Once you have gone to Hogwart's, gotten your wand, gotten sorted by the sorting hat, and so on, you're able to hop back to the alley any time you want to go on a shopping spree. Here are some of the things you can buy with your coins!

Owl Shop
You can buy "fun changes" here. They don't change the gameplay, but they make it fun to play. So for example one extra is "Carrot Wands". The wands act exactly like they should, but they happen to look like carrots. These can be quite funny!

Silhouettes
Ice Rink
Disguise
Carrot Wands

You can also buy gold bricks here, if you really really can't get through a level. They're expensive though. 100,000 for the first one, 200,000 for the second one, and so on. I highly recommend you simply earn them the old fashioned way!

Wizarding Supplies
You can buy a fun spell here if you want to try something new

slugulus eructo 500000
accio 4 mil
engorgio skullus 10,000
stupefy 150000
trip jinx 40,000
flipendo 125000
incarcerous 100000
glacius 110000
herbifors 15000
anteoculatia 95000
calvorio 20000
colovraia 75000
redactum skullus 10000
lovomoyot motyid 30000
tarantallegra 75000
entomorphis 80000
rictusempra 65000
multicorfors 50000

You can also enter codes if you find them.

Lego Harry Potter Walkthrough