Formulate a list of your options for post-college life before you're bombarded with questions. If your prospective career will eventually require an advanced degree, consider going on to graduate school. Entering the workforce is a common choice, though finding that first "dream job" is not always easy. Other options include joining the military, traveling, doing an internship or volunteering.
Respond verbally to the question honestly without feeling the need to explain yourself. People are curious about your next step, so just give them a brief, general idea of your plans. If you don't have a job lined up, simply say, "I'm busy pursuing my career options and am enjoying some time off." You will find a job eventually, so enjoy the time off while you have it.
Compose your response in advance if you're taking time off to travel or volunteer. When people ask you questions about your plans, say something like, "I've worked hard the past four years and plan to explore the world a bit before settling into the workforce."
Develop an answer if you've been lucky enough to land an exciting first job or are pursuing graduate school. When people ask about your plans, you can tell them that you've received an offer from [insert company] as a [insert job title]. If you'e going to graduate school, say where you're going and what you'll be studying. Mention that you're excited about the opportunity and leave your answer at that.