I have used the command strips that have the velcro on them to great success for anything with a flat back. Fairly heavy 8x10 picture frame, used four, one in each corner of the frame. Small plastic clock, used two on the battery box. The velcro presses in very well and we leave them hanging all the time, even when traveling. Two trips and have not fallen yet. Have checked and they didn't even move enough to re-press the velcro.
I have used self tapping screws about 1" in length if it's flush mount (screw head all the way to the wall) or 1.5 " self tapping if I need to leave 1/2" of the screw out of the wall to hang something. If it's fairly heavy, I definitely find a stud. I have two stud finders, one was useless, the other does a great job identifying where the studs are. It even works from the outside through the fiberglass.