The Helsinki Daytrip is nice. The ferry is huge with plenty of places to eat, rest and shop. Tallinn and Riga are both nice capital cities with great old towns. Riga has more natural parks / green space and may be slightly less expensive, however both are great compared to main European tourist cities. The town on Tartu between the two cities is worth a stop. It has lots of nice restaurants, river side cafes and a university with nice hill side views of the old city. Vilnius is also worth a visit if you have the time.