All are grammatically correct, and there is little practical difference in their meaning. Noting only the words that change with each example,

>"had been up to these days" indicates the actions are in the past, although recent;

>"was up to these days" could indicate the actions are in the present and/or near past;

>"has been up to these days" indicates the actions are in the near past;

>"is up to these days" indicates the actions are presently happening and ongoing.

**"These days"** keeps all of these examples in the present, recent or near past.