Verb tense when comparing two different time frames in a sentence
"Then and now" usually implies the same action or state of being ("I've known him for twenty years and he's kind of a jerk, then and now"). To contrast different ones, you should replace the word "and" with the word "versus":
"I was doing different things then versus now."
Or separate the "then" part from the "now" part:
"I am doing different things now than I was then.
In both cases, however, your verb agreement is determined by whichever time (past or present) comes first.