Short trip suggestions? St.Petersburg, Helsinki... (Tallinn)

Hi all,

Just wondered if anyone has done the Helsinki/Tallinn to St. Petersburg trip via train, and wondered if you think it's 'do-able' in a day? Or would you suggest an overnight stop?
Train travel seems quite expensive to St. Petersburg; I've checked a few websites but the most popular seems to be to buy tickets from here via the Allegro train? - Protected content
(additional info here if anyone else is looking in to this: Protected content )

Can anyone recommend any other good trips worth making for short weekend breaks? I'm hoping to visit Stockholm & Riga also when I next have a weekend free :)

Thank you!
Stephanie