应该不可以 .
以下是查韦氏字典得到的解释：
on--used as a function word to indicate a time frame during which something takes place
in---used as a function word to indicate limitation,qualification and circumstance.
所以,句子的意思是“这几天里你在做些什么?”应该用on表示the things you have done during these days,是比较具体的时间.而in表示一种限制,是比较笼统的时间.these days 属于比较具体的时间概念.