Skydive into a active volcano without a parachute.
Try to kill X predator/soldier while unarmed and unarmored.
See what every substance on earth tastes like in both solid and liquid form.
Ride a tornado.
Similarly, ride a hurricane.
Movie stuntman so they can skip having to fake a bunch of stuff.
Test dummy for weapons tests or crash tests.
See if it is possible to break your own neck.
Spend a while trying to see if you can randomly whip up useful medical concoctions by mixing stuff together and trying them.
See how far down you can make it into a cave or in the ocean before the environment kills you.
Try to freak people out by repeatedly dying in the same spot in the same way, such as breaking into a restricted area and getting shot, then doing it again next time you get up.
See what happens if you stand next to a radiation source until you die instead of only brief exposure.
Volunteer for 'playing' with people who are into extreme kinks they can't do due to them being incredibly dangerous or fatal.
See if you can die from sensory overload.
Play dodgeball, but with archers on the other side.
Try to tame every wild animal until you succeed.
Hardcore training: use practice weapons in spars against people with real weapons, repeatedly, until you become an expert.
Tons of options, though they might not seem 'fun' if you haven't gotten to the point of extreme boredom, or are already an extreme masochist.