Last trip I was lucky enough to do the Scavi tour. I applied for a place about three months before hand. I also pre-booked the Galleria Borghese as I had heard that it sometimes sells out.
Is there anything else that is worth pre-booking a few months prior to visiting Rome?
I'm less concerned with the 'skip the line' kind of pre-bookings that can be done a day or two ahead... more the sort of thing that books out months ahead, or you can only visit via special arrangement.
Scavi was just so incredible I'm keen to find other hidden gems missed by most people due to the extra planning time involved.