We visited both Helsinki and Tallinn in June. Yes, Tallinn is doable as a day trip - many do just that. We, ourselves, spent two nights there and were content with that decision - but if all you have is a day, then go rather than miss it. Take the catamaran rather than the slower ferry.

Tallinn's sights are concentrated in the Old Town. There is a combination bus tour of the town and walking tour of the Old Town available from the Tourist Office (although you join the walking tour some distance from the Tourist Office). You can take just the walking tour, and should do at least that.

The Old Town is vibrant with restaurants, shops, and galleries. If I return, I will be much more serious about visiting the galleries. We bought a spectacular silk wall hanging for a ridiculously affordable price and got to meet the young artist.