need help with Europe trip schedule

Sorry, I can't type Chinese in this computer.

 I have booked a 18 days airline tickets by using my mileages.

This is my schedule:  Rome 4 days.  Florence 1 day,  venice 1 day  milan 1 day  ,  paris 4 days, london 3 days,  North Ireland 1 day .  ( here is total 15 days, which include the transpation time from one location to another).  

Besides the first day and last day at air port, I still have one more day to spend, I was thinking Florence , Venice or Milan, but not sure which one should add one more day. 

Thanks in advance