Suntory puts out a large number of great drinks that can be enjoyed all year around. Not too long ago they started offering a 9% Chu-hai with a stronger citrus flavour called ダブルグレープフルーツ, or "Double Grapefruit". This has been one of my favourite drinks to enjoy when sitting in the park or taking a Shinkansen home after a day of meetings in Tokyo, as it has just the right balance of flavour and alcohol.
A 500mL can generally sells for a little over 200 Yen at the local convenience store or, if you're near a grocery store, cans can be found for as little as 138 Yen. I generally pick these up at a nearby Apita along with some sort of sweet bread to compliment the strong citrus taste.
As with all alcoholic beverages, please do not drive after having one of these. For me the buzz from a singe can becomes noticeable within 20 minutes and lasts a good two or three hours before fading.