"The lake" was a lake that Marty McFly and Jennifer Parker were planning to camp out at before their adventures with Doc Brown, had only Marty had a vehicle like the Toyota 4X4 he saw downtown. When Marty changed the past, it affected the future and the 4X4 was his. When Marty and Jennifer left for 2015, his family assumed that they left for the lake. When Marty returned home after 1885, they thought he picked up his western wear at the lake.
While in 1885, Doc Brown may have been refering to the same lake to wait for it to freeze over in winter. Furthermore, Buford Tannen and his gang were sleeping next to a lake that could have been the same.