"Certi giorni sono più felice."
I found this one confusing because the verb 'sono' can be used with both 'io' and 'loro'; I didn't know if I was happier or if the certain days themselves were happier. Perhaps in Italian only people can be called happy, not days?
Then it is obviously "io sono più felice" :)
"Certi giorni sono più felici" would have been "Some days are happier"