Weather permitting, Alton Bay is used by about 50 seaplanes each month. But come the freeze, conventional aircraft come in droves. It’s been that way since the 1960s, and the runway will see more than 100 planes from throughout New England on particularly busy days. (The average is closer to 10 or 15 a day.) There are restaurants and shops nearby, but landing on a runway made of ice may be the main draw.