What can i do in less than a day in Tallinn?

Hello Eesti,

I will be traveling to Tallinn for 2 days in September and will have a few hours to spare and go around town.

I will be having Sunday and Monday afternoons free. Sept 9th-10th should it coincide with any cool local events.

What are some must see/do things while there?

Thanks.

EDIT: Well it turns out i will be moving in Tallinn in about a month as i just got a job there. Your ideas were very helpful. I got to see the Seaplane Harbour, The Architecture Museum, the Communist Exhibition and the Old City. Also i had lunch at the Aed von Khrali. Loved the rhubarb cheesecake :) What i didn't get to see, i will most definitely in the future. Thank you all guys for your help!