We spent most of a day in Helsinki a few weeks ago, as a stop on the RCCI cruise, right after St. Petersburg. (It's a fine cruise, though you'll be tired after five consecutive port days.) The cruise line provided a shuttle into the center, a good value we thought. We walked up the Esplanade to Market Square, stopping for coffee at Kapelli which was very nice (and had restrooms). The TI is now in the city hall, about a block east of where the RS guide shows it. Restrooms there. Market Square was filled with booths selling food and stuff, looked more for tourists than for locals. We took a look at the Orthodox Cathedral, would have gone in if we'd had more time. The Lutheran Cathedral on Senate Square is very good. The city history museum was a disappointment to me (but restrooms again). Then back along the Esplanade and up to the train station which was very interesting and has perhaps the best-looking Burger King I've ever seen, also restrooms (need coin). Then north past the Mannerheim statue and the Music Centre (didn't go in) to the National Museum, very well done, could have spent more time there (again restrooms). I missed the Church in the Rock because I had trouble finding it and got nervous about time.
Basically an easy on-foot self-guided day (with help from RS guide) in an interesting and prosperous-looking city. Highlights for me were the Lutheran Cathedral and Senate Square, station, and National Museum. Lesser lights were Market Square and the city museum. Lots of sidewalk eating on the Esplanade, and cafes elsewhere. Interesting architecture. Have a good visit!
EDIT -- Rereading the thread I wonder why I seemed to obsess on restrooms above, I think it's from another thread on this forum where someone was anxious and I projected onto this OP. Not a sign of any pathology or fixation on my part, of course! ;-)