If you're going to St. Petersburg, try to hit the Hermitage (priceless masterpiece works of art throughout), Peterhof Palace, Catherine's Palace and the Church on Spilled Blood - all were spectacular. Also, try to get to the ballet in the evening if you have time. If you want to do all this, book them early because they fill up fast.
In Germany, I'd try to take the trip to Berlin - it was well worth the bus ride.
In Denmark, try to take the trip to Rosenborg Castle to see the Crown Jewels.
From Dover you might want to take the trip to Leeds Castle - it was very nice.
Retired in 2006 at age 49.
"Who among us is smart enough to learn from the mistakes of others?" - Voltaire