if i want to say one product out of stock, can i put this way? this product is temporarily unavailable? not available? both correct?

about London.

hi, everyone, I want to ask someone whoever been to London, there are so many options to visit. because this is my first time go to London, which is a must see? for examples: Westminister Abby / Brith Museum / St.paul cathedral / national portrait gallery /tower bridge /national history museum ...? oh.because my time is limited,  so i just want to choose some of these.any suggestion? 

thank you.

spend or spent hours? I have a grammar question: spend hours to write? spent hours to written?