Antonyms for the phrase "plunges in to" could include withdrawing from, avoiding, hesitating, or delaying. Alternatively, one might "gradually enter" a situation instead of plunging into it, or "approach carefully" instead of diving headfirst. The opposite of plunging into an activity might be taking a step back or considering other options before fully committing. While there are certainly times when jumping in with both feet is necessary, it is also important to weigh the risks and benefits of doing so, and to consider different approaches to a situation if necessary.