That certainly is true for Frederick, which might be one of the best places in the country to plan a summer “staycation.” This is a city that likes to have fun year-around, but it really heats up — both figuratively and literally — after Memorial Day and keeps going right until school starts again. Alive@Five, the summer concert series sponsored by the Downtown Frederick Partnership at Carroll Creek Park, is underway, attracting good-size audiences to a variety of music on Thursdays.