For me, the answer to this question is: parts that involve no architectural decisions, and that won't need to be extended or built upon significantly in the future.

When I'm working on a greenfield project that I intend to build out further (which is what I am currently doing), I find that there's not a lot of work that fits those criteria. I expect that can change drastically when you're working on something that is either more mature, or more narrowly scoped (and thus won't need to be extended too much, meaning poor architectural decisions are not a big issue).