Citing sickness as the reason for your absence might seem trite and overdone, but it's also effective. It can't easily be proven that you weren't in fact sick, unless your professor spots you at the beach later that same day. As for the sickness itself, come up with something typical and undramatic (no terminal diseases.) Even the strictest of professors won't ask questions if you say you were suffering from some unfortunate digestive problems.
If you want to take next Friday off to make plans with your friends, let your professor know a week or so in advance that you won't be able to make it to class that day. Do this by e-mail if possible. Professors are quite busy, and are likely to ask fewer, if any, follow-up questions by e-mail, than they would in personal conversation. Foreshadowing your impending day of absence is a sure-fire way to avoid scrutiny for missing class, and it keeps you from having to tell unnecessary lies.
Colleges and universities often have a lot going on. It's not unlikely that there might be an exciting sports game, a fundraising event or an extracurricular club meeting happening on campus all in the same day. While many professors don't like to hear excuses for skipping --no matter what the reason-- citing an academic reason, like attending an intriguing guest lecture, softens the blow and makes you look responsible.
Professors who have taught for any length of time have heard all the excuses in the book. Don't insult your instructor's intelligence by lying or making up something that makes you look irresponsible or foolish in the process. An honest, no-nonsense approach will do just fine in many cases. Simply say, "I was stressed, so I took a day off." An honest approach goes a long way in smaller schools where absences don't go unnoticed.