Does Ekadasi Upavasa mean that we have to fast the entire day, or is it okay to take in something (except rice)? I drink coffee every morning and evening. Does this spoil my Upavasa?

There are different grades in observing Ekadasi Upavasa. Take one meal on a dasami day, fast completely (without even a drop of water) on Ekadasi day, and take one meal on the Dwadasi day. In this, if the Shravana Nakshatra falls on the Dvadasi day, then you should observe complete fast on that day too. On the Ekadasi day, you should not sleep at least until midnight. This is the most ideal.
Anything less than this is okay, as long as you don’t consume rice on Ekadasi day.