Group sex with your parents and your parents parents.
If one or more of the participating parents is dead and
requires exhumation before participating then 17th base
becomes: 103453rd base,103454th base,103455th base,103456th base,103457th base,103458th base,103459th base,103460th base,103461st base,103462nd base,103463rd base,103464th base,103465th base,103466th base,103457th base depending on the combination exhumed and then group
Me: "While I was at grandma's place she suggested we spice things up a bit and move from boring old 3rd base to full on 17th base."
Friend: "Dude, isn't that pretty far out even for you?"
Me: "Yeah but at least they are all alive, you know."