The phrase "was up to" refers to the actions or behaviors of a person in a particular situation or time frame. There are several synonyms that can be used instead of "was up to" such as "was doing", "was engaged in", "was occupied with", and "was involved in". These synonyms can be used interchangeably depending on the context and tone of the sentence. For instance, if you want to ask someone about their recent activities, you can say "What were you up to?" or "What were you doing?" Similarly, if you want to describe someone's involvement in a particular task, you can say "She was engaged in the project" or "She was occupied with the task".