I live by the Tallinn in Your Pocket Guidebook that you can get in any bookstore (Raamat) here in Tallinn. It hasn't steered me wrong yet.
My personal favs:
Old Town - anywhere! Outside the city wall in the parks by the Balti Jaam (train station), or through the stone-covered winding streets, browsing in the shop windows or stopping at a cafe for a bite. The various old churches, the towers and the views make this my all time favorite place to be.
Controvento - Italian restaurant in Old Town, not cheap but very delicious.
Old Hansa - okay, major tourist trap and not a place where Estonians actually eat, but a very fun dining experience. It's a must do if you come for a visit.
Nukuteater - a live theater / puppet show, shows are rated by ages and a great place to take even little kids.
Estonian National Theater - ballets, operas, etc. Very well done and so enjoyable.
Pirita/Kadriorg - beautiful walks along the oceanfront, a beautiful park area, museums and just a nice place to be outdoors. Also outdoor ice rinks can be found further up from this area.
The Open-Air Museum or the Occupation Museum - both really excellent history museums, both completely different and both worth visiting. (the open-air is better for kids).
Juku - indoor children's playgrounds found in several locations around Tallinn and a must for those with energetic kids, especially in the winter.
Laulupidu (sp?)or the Song Bowl - a good place to sled in winter, also has an indoor area where fairs are sometimes held and of course the location of the Olympics of Song and Dance (ie The Estonian Song Festival which only occurs every five years).
The Saint Martin's Day craft fair - held in the fall, this was the best place to find a very large selection of less expensive handcrafted Estonian items and food for sale. Also had musical performances and areas where you can see how the handcrafts are done and for 20-40 kroons try it (great for the kids!). This year it was held at Saku Suurhall (Arena) and was really fun.
Christmas in Tallinn - the christmas lights are especially nice here since the daylight hours are very short at this time. In Reakoja Plats (the main square in lower old town) wooden huts go up and each evening just before and after Christmas day and food is and handcrafted goods are available to purchase. Santa Clause visited at some point near Christmas and a huge, lighted tree stands in the center of it all. It was a great place to congregate, feel festive and have some fun.