As always with CEE, it is worth checking out the local
In Your Pocket guide for the latest information.
I have visited Tallinn twice, a few years ago now. It is a very compact city, easy to walk around. Sup local beers on Town Hall Square, explore the narrow medieval streets around it, visit the castle and the imposing Russian Orthodox Church. Outside the old town, there is not much to see (unless you are a fan of Soviet apartment blocks).
On the down side, since EU entry and the arrival of the lo-cost carriers, Tallinn has developed a reputation for attracting loud, boozy stag nights from the UK for a cheap weekend away. When I was there 10 years ago
there were some great little bars in cellars and courtyards. I can only assume there are more (for better or worse) now.
Anyway, I am sure you'll have a great time with the long summer evenings. And no doubt it well be refreshingly cheap compared to all the other places you visit in Scandinavia (hence all the Swedes and Finns on shopping trips there).