I was in Tallinn this morning - we were there for three days. We loved Tallinn (and the other places in Estonia, and Latvia, we went to). Not sure if the Tallinn card is worth it, but we didn't bother with it, reasons being 1) most things are fairly inexpensive there, relative to much of Europe, and 2) for us, the main attraction was the city itself - it's architecture, old walls, towers, etc. we didn't find any of the touristy things very compelling. Must-see items? I'd say this: the city gets crowded with tourists from cruise ships. They start showing up at 8 am and are there until 5-8 pm. Outside of those hours the place is at its best. If you want to see the center without big crowds, get up early and be out there exploring the city from 6-8 am. Be sure to get to the upper town, which has some beautiful views and is less crowded.
I would be remiss if I didn't suggest that Latvia is an under-appreciated gem, too. Tallinn gets more attention and tourists too, but we loved all three of the Baltic countries. Riga is delightful (be sure to give it a few days, too), as are other parts of Latvia (don't miss Rundale Palace). Plenty to see and do all around this region.
We are still traveling (currently in Poland) so I won't be online much until we get home next week. You will like Tallinn, it's cool.