Vyborg (Russian: Вы́борг; Finnish: Viipuri; Swedish: Viborg) is a city in Leningrad Oblast, Russia. Located near the Finnish border, it was in fact Finland's second-largest city until World War II, when it was handed to the Soviets as war reparations.
Bus excursions depart St. Petersburg's Gostinyy Dvor on weekend mornings and cost about 700 Rubles, returning to St. Petersburg in the evening, a total of about 10-11 hours.
By train from St. Petersburg, express trains and local "elektrichka" trains depart from the Finlandskiy station. Express trains cost 182 Rubles each way, and tickets can be purchased in advance or at windows 27-30 at the Finland station (not in the main hall) on the day of departure. Beware of long lines if attempting to purchase tickets shortly before the train departs. Elektrichka tickets are slightly cheaper for a longer ride and can be purchased in the main hall at Finlandskiy station on the day of travel only.
Express trains (with upholstered seats and a restaurant car) travel on the following schedule:
Elektrichkas (with wooden bench-seats and few amenities) run more or less throughout the day, and a sampling of their schedule is:
The major international trains  to Helsinki stop in Vyborg, although they are relatively expensive. Eg. since late 2010, the high-speed Allegro train travels between St. Petersburg and Helsinki, stopping in Vyborg .
Vyborg is compact and walkable. Those in a hurry can take local bus #12 from the train station to the castle. City maps of Vyborg can be purchased at bookstores in St. Petersburg, at news kiosks at the Vyborg train station, and are sometimes sold at the Vyborg market ("rynok").
A walking tour of the major sights of the city can be done in about three to four hours, with Vyborg Castle being the highlight of the tour.
From the train station, walk down Leningradskiy Prospekt and turn right along the embankment of the Salakka-Lakhti Inlet. See the Market Square on the left at the end of the quay, where one can see the Market Hall or "Rynok" and the 16th-century Round Tower. Walk past the Round Tower to the Lutheran Peter and Paul Cathedral in the park. Cross the street to see the blue-painted Transfiguration Cathedral, then walk up Vyborgskaya Ulitsa past the Council House Tower along the remnants of the old city wall, then head up the street (this is the least picturesque part of the walk through the Stone City) to see the Clock Tower tucked in among the apartment buildings. Before the Castle Bridge, see the Statue of Torgils Knutsson, the Swedish knight who founded the city in the 13th Century. Look across the gulf to the Statue of Peter the Great in the park opposite.
Cross the bridge to Vyborg Castle which was originally built in 1293 by Swedes, captured by Peter the Great in the 18th Century, served as a Finnish prison in the 19th Century, and passed between the hands of the Soviets and Finns multiple times during World War II. The castle has several different exhibits, each with its own entry fee, and one can climb to the top of the tower for a nice view of the city. (Tickets for the tower cost 80 Rubles from the museum cashier.)
Cross back over the bridge and walk up Krepostnaya Ulitsa through the town, turn left on Suvorovskiy Prospekt, see the Alvar Aalto Library and the Statue of a Moose in the park, then proceed to Red Square and the Statue of Lenin before heading up Vokzalnaya Ulitsa back to the train station.
Visit the market hall ("Rynok") and the market square outside, where one can find Karelian knits and woolen clothing, as well as tablecloths and lace among the fruits, vegetables, shoes, and other sundry items. There is a public toilet available in the market, but it is very basic indeed.
Slavyanskaya Trapeza in the basement of Ul. Yuzhniy Val 4, just across the bridge from Vyborg Castle, serves moderately-priced Russian food, including excellent Chicken Kiev for about 200 Rubles.
Restaurant NiKa at the Hotel Atlantik (Ul. Podgornaya 9). Prices are moderate to high.
Y Borharda  right across the street from Hotel Atlantik (Ul. Podgornaya 10). Quite an extensive menu, including an excellent solyanka. Moderate to high prices in this rustic restaurant with kitsch decorations.
Kruglaya Bashnya, a Russian restaurant inside the Round Tower. Not as expensive as one might expect for its prime location, but service is slow.