While no one is technically forcing you to do work over the summer, you'd be foolish to take extended breaks.
One of the benefits of a tenured faculty position at a research university is that you get great flexibility in your schedule. Today, I left work early (e.g., 4pm) to pick up my kids at summer camp and go to the park. I have no (or almost no) formal obligations over the summer.
That said, this evening, I was doing data processing and reading through papers. It's now 1 AM.
My long answer to your question is that it's certainly possible for a research active faculty member to take a whole summer off, collect summer salary, and pick up in the fall with research and teaching.
This sounds like a very bad idea.
Right now, funding levels are so competitive, that everyone I know with research funding is trying to be as productive as possible throughout the year. The summer might be a bit more laid-back, and I bet most research faculty spend less time in the office or lab. Most will probably travel to at least one conference or workshop, and I bet most also take a week or two of "real vacation" with family or friends.
But, if you're not working on papers or grants year-round, you're probably falling behind and will likely have difficulty getting that grant (and summer salary) renewed.
This summer, I helped submit a major center grant, I'm drafting another grant for my group's research, and I'm trying to finish up ~3 papers. Add in some papers and grant proposals to review as service. I'm not trying to boast - I suspect this isn't an unusual summer work load for research-active faculty.