In the question: Do you usually drive to work?
we can say: Yes, but I'm taking the bus today.
(present continuous for sth preplanned)
Can we answer " Yes, but I have taken the bus today." ?
(present perfect for an action in the past when we don't know exactly the time)
Yes, but I took the bus today.
Yes, but I took the bus today and yesterday.
For a longer specific period of time, then I would use the past perfect:
Yes, but I've taken the bus every day for the last week.
(Note: In BrE, we usually say "to catch the bus" so "I caught the bus today" or "I came on the bus today".)