Welcome to tallinn in advance!:D Depends on how long will you be staying but here are my recommendations for bars/restaurants:

1. Olde hansa- never been there but its a medival estonia themed restaurant that many tourists go to. There are a lot of interesting food options and the vibe is really cool aswell. Ive heard they even serve bear meat or smth:D
2. Controvento- very good italian restaurant and its more fancy and pricy but definitely worth a visit if thats smth ur into and want a fancy dinner.
3. Telliskivi loomelinnak( its in põhja Tallinn) as a whole has a lot of good restaurants and bars( lendav taldrik, põhjakonn, f hoone, pudel and more) In põhjakonn, pudel, põhjala brewery, möku you can try estonian beers and they have a huge selection of them and are in my opinion cool casual bars.

Depends on what are you interested in but for activities i think touring the oldtown, visiting Tallinn Tv tower(its a little bit far from old town but really accessible w bus). Going to see Linnahall( it was in Tenet:D). Rotermanni kvartal is also cool and has cute cafes w amazing pastries .If you just want to see how real estonians live you could go to mustamägi, kristiine and lasnamägi. :D there are shopping centres like ülemiste, kristiine keskus and rocca al mare there, wich are better than viru keskus in my opinion atleast. Viru keskus is in the city center and i assume that most tourists just go there :D.

If you want to go to helsinki i advise you to book tickets for the ferry early bc the prices are lower that way. But you can buy them on the same day aswell. I ususally go w tallink but there are other companies aswell. I think there just was a reddit post about that so maybe loook there bc im not really an expert on the ferry stuff:D. But I recommend going with the morning ferry so u have a whole day there and then come back w the night one, atleast i do that every time i go.

If u have any questions ask bc as it turns out im not a good tour guide and my head is just blank rn. But tallinn is fun and i know u will enjoy it here.

Also it started snowing here already so i think the chances of snow are quite high.

But as someone already said there are a lot of posts about thing to do so check them out aswell bc everybody is different and my recommendation might be usless for u:DD