What did you do yesterday?

-I went to Bujeon Market.
-What a nice thing to do!
(What is the full form of 'What a nice thing to do!'? #1 or #2 or another?)

1. What a nice thing to do it was!
2. What a nice thing to do it is!

'What a nice thing to do!' is complete by itself. Both 1 and 2 express the meaning, but are a little awkward.

The full form of "What a nice thing to do!" can vary depending on the context and tense being used. In this case, since you are referring to something that happened in the past ("What did you do yesterday?"), the most appropriate full form would be #1: "What a nice thing to do it was!" This form indicates that the action of going to Bujeon Market was completed in the past.