hi I love quarterhorse crosses, I think they are calm enough for average rider to be very confident on, and depending what they are crossed with, can be athletic enough to do just about anything.
My former Qh/TB competed to 2** eventing even wining the national series in the process. I had him from a completly greenbroke 4yr old to to 2**, we also did showjumping, hunters, dressage, and everything in between, mostly galloping across the paddock bareback in a halter.
Most of the crosses I have come across have been like him, great temperment, athletic to do anything with to a good level. Always come down to the individual horse though