We just returned last night from a wonderful visit to some excellent ports. We chose this itinerary because it stayed overnight in St Petersburg and included Riga. Every port was charming, and St. Petersburg is definitely worth the two days.
Helsinki - we took an overall city tour through RC. Good information and time for shopping. We chose to return with the bus, but could have stayed longer and returned by shuttle.
St. Petersburg - we chose a 2 day private tour through Best Guides. Fabulous and worth every penny! Our guide, Kirill, was excellent and graciously answered our questions. You can definitely spend more time in SPB.
Good location and view.