I guess "use only twice" means I get to travel through time and then use the second trip to get back to the present if I want to or "retire" in some other time.

No question I would set the time machine back to 1963 and live back then until 1969. I would collect all 97 missing episodes of Doctor Who and whatever other lost media I could get my hands on. After I watch the moon landing live on TV I would return back to the present.

No, I wouldn't try to prevent the Kennedy assassination. The movie Running Against Time has already taught me that's a bad idea.

If "use only twice" means I get two full round trips in a time machine then after going back to the 1960s I would head 22 years into the future to the year 2040 and gather as much information as I can about what will happen in the upcoming two decades and end my journey in 2009 to manipulate bitcoin prices until the present time.

Wait a minute...Is that already happening? Do I get my hands on a time machine someday? Hhhmmm...