Cycle trip to Trakai
Getting to Trakai is easy: there is a frequent (every 30 min) mini-bus service or a much less frequent (every 1-2.5 hours) train service. Trakai is perfect for cycling, so rent out bikes in Vilnius and then take the train (€4 return per person, yes really!). Bike rentals are €10-15 from Velo City, just close to the station and return time can be arranged for late evenings!
In Trakai you can cycle around the shore of the lakes, and it really goes almost all around the island! On weekends you will be sharing with the strolling crowds, but you don't want to go too fast anyway, to enjoy the surroundings. We were recommended to get off at a nearby city, Lentvaris, and cycle to Trakai from there, but it's really not worth it, unless you want the extra exercise. Our cycle path is included for guidance:
Once by the castle, it is nice to rent a boat to paddle around the lake. The boats are €6-8/hour, but they don't have a wait list, so you'll have to hang around for a bit until a boat becomes available. Depending on your ability, you can chill leisurely paddling around the castle, or you could even try to make it closer to the white palace all the way on the other side of the like, behind the castle:
An absolute must is to try "kibinai", which is a traditional pastry filled with mutton, or any other meaty or vegetarian filling you choose. The most authentic place to my knowledge is "Senoji Kibininė", but, apart from kibinai themselves, recently I've had quite appalling food and service there. Also, try it with the cold beetroot soup, also known as "pink soup" - it's very refreshing on a warm summer day.
Finally, if you have some spare time and energy left, you should visit the castle itself that is home to an elaborate exhibition about medieval Lithuania. If musems are not quite your thing, take a stroll along the lakes. If you want to find a more quiet spot with a view, you might even want to cross the bridge to the other side, or else walk down to the more residential areas.