Ferry from Helsinki to Tallinn- spend just a day in Tallinn or 2?

Tallinn is certainly worth two days. I am a retired, slow traveler, and I spent 11 days in Tallinn in 2023. These are some of the places in Old Town I had on my list of sightseeing targets, organized semi-geographically rather than in priority order:

• Vanalinn (Old Town) Well-preserved medieval center with cobblestoned lanes, gabled houses, historic churches, and turreted city walls (most picturesque on west side) with 26 watchtowers. Upper (Toompea) and Lower sections. Mobbed by cruisers from mid-morning. Pickpockets.

• Toompea (Upper Town): Fabulous views of Lower Town. Most atmospheric approach through gate at foot of Pikk jalg.

• Aleksander Nevski Katedraal (Russian Orthodox Cathedral)

• Danish King’s Garden has great view of Lower Town.

• Toompea Loss (Castle): Art and craft exhibitions

• Piiskopi, Kohtuotsa and Patkuli viewing platforms have good views. Patkuli probably less crowded.

• Tallinna Puha Neitsi Maarja Piiskoplik Toomkirik (St. Mary’s Cathedral, Dome Church) and Tower: Gothic; rebuilt after 1684 fire. Sometimes has concerts.

• Ruutelkonna hoone (Knight’s House): 19C Neo-Renaissance.

Lower Town part of Old Town

• Okupatsioonide Muuseum (Vabamu Museum of Occupations and Freedom), Toompea tanav 8: Translated printed explanation better than audio guide. Seven 30-min. videos. Excellent introduction to Estonia's 20th-century history.
• Kiek in de Kok Tower, Komandandi 2: Views; current exhibits unclear. I skipped this.
• Neitsitorn (Maiden Tower): Good views; exhibits so-so.
• Tallinn Art Hall
• Tallinn Art Hall Free Gallery
• Adamson-Ericu Muuseum: Art and decorative-art museum, branch of Art Museum of Estonia; incl. ceramics, jewelry, textiles and furniture
• Niguliste Muuseum (St. Nicholas Church): Gothic ecclesiastical art in restored Gothic church
• Linnamuuseum (Fotomuuseum): Photography
• Tallinna Raekoda (Town Hall) and Tower: 15C Gothic building with history museum
• Raekoja Plats: gorgeous square.
• Disaini- ja Arhitektuurigalerie: Exhibition of Estonian product design.
• Viru Gate: Picturesque access point to Old Town.
• Dominiiklaste Kloosti Muuseum: Extensive collection of medieval and Renaissance stone-carving.
• Katariina kaik (St. Catherine’s Passageway): Medieval alley with artisans. Connects Vene and Muurivahe streets near Viru Gate.
• Puhavaimu Kirik (Holy Spirit Church): 13C; splendidly preserved with medieval and Renaissance features
• Eesti Ajaloomuuseum (Museum of Estonian History), Suurgildi Hoone (Great Guild Building): High-tech exhibits and videos
• Pikk Street and Lai Street lined with interesting period buildings.
• Issandamuutmise Kirik (Ch of the Transfiguration of Our Lord): Extraordinary iconostasis. Hours unknown. www.teelistekirikud.ekn.ee
• Tarbekunsti- ja Disainimuuseum (Museum of Applied Art and Design), in courtyard behind Lai tanav 17: Includes textiles, jewelry, glass and 18C-20C ceramics. Impressive collection.
• Kolm Ode (Three Sisters), Pikk 71, just beyond Oleviste Kirik. Three multicolored 14C merchants’ houses, beautifully renovated, now hotel.
• Meremuuseum (Marittime Museum), Fat Margaret Tower: Good reviews, but I skipped this. Great views from top of tower.
• Paks Margareeta (Fat Margaret Tower)kk 70: Houses Meremuuseum. Gorgeous views from top of tower.
  

I'll put the sights outside Old Town in a separate post.

Other Tallinn sights, outside Old Town.

• Eesti Kaasaegne Kunsti Muuseum (Estonia Contemporary Art Mus, EKKM)
• Kalamaja: Bohemian district on the coast just west of Old Town, mainly residential with traditional wood houses. Now one of city’s most interesting neighborhoods (LP).
• Lennusadam (Seaplane Harbor): Impressive boat/plane museum.
• Juhan Kuusi Dokfoto Keskus (Juhan Kuus Documentary Photo Center)
• Fotografiska Tallinn: Smaller sibling of large, privately operated photography museum in Stockholm. Pricey but can be worth it if the current exhbitions are of interest. Telliskivi: Interesting modern area north of RR station; now alternative shopping and entertainment district.
• Eesti vabaohumuuseum (Estonian Open-Air Museum): Authentic 18C - 20C farm and village buildings (incl wooden church) in forested parkland. Nature trails, some performances. Best to rent audio guide. Outside town.
• Klooga Memorial: On site of Klooga concentration camp.
  
• Open Air Holocaust Museum, just north of Klooga Memorial. WSW of Tallinn, not too far from Paldiski.
• Hotel Viru: Old communist hotel with tour describing how visitors were monitored by KGB. Kitschy but fun. The tour also goes to top floor for great views over Tallinn. Photography hint: smoking balcony one floor down has stunning views of Old Town.
• Rotermann Quarter (Rotermanni Kvartal): Cutting-edge architecture in area where industrial buildings are being repurposed. I found good restaurants here.
• Eesti arhitektuurimuuseum (Museum of Estonian Architecture): 19C-20C drawings, models, and some (mostly 1920s/1930s) furniture in former salt-storage warehouse.
  
• Kadriorg Park: Seaside park with Kumu Art Museum, palace built by Peter the Great and palace gardens.
• Mikkeli Muuseum, Kadriorg Park: Small museum with art and decorative art (Asian porcelain).
• Kadriorg Palace – Kadriorg Art Museum, Kadriorg Park: One of N Europe’s best Baroque palaces. Collection is more modest.
• Kumu Art Museum, Kadriorg Park: Excellent collection of 20th-century Estonian art.
  
• Japanese Garden, Kadriorg Park

There are other points of interest, but they are well outside the center and really not practical for short-term visitors.

As you can see from acraven’s info, there is a lot you can see. I spent a week there in September and could easily spend another week. 2 days would give you a fast overview. It’s a pretty town and two days would go fast.

I spent 6 nights in Tallinn and had plans for a second visit that got preempted. But my favorite memory of Tallinn was dinner and a drink in the town square, and spending the evening relaxing on their summer pop-up seating. It was like a super chill town slumber party.

Spend at least the evening.

I would the spend the night there. As others have already posted: there’s enough to see and do. The ferry trip is very nice (relaxing). We choose to travel with Tallink Silja Line.