one solution could be

irb(main):210:0* class Range
irb(main):211:1> def rand
irb(main):212:2> return self.begin + Kernel.rand(self.end-self.begin)
irb(main):213:2> end
irb(main):214:1> end
=> nil
irb(main):215:0> (-10..10).rand
=> 2
irb(main):216:0> (-10..10).rand
=> 6
irb(main):217:0> (-10..10).rand
=> 4
irb(main):218:0> (-10..10).rand
=> -1

it's a matter of style to write

rand(-10..10) or (-10..10).rand

but since 10.sin doesn't work, as opposite tosin(10)
non object oriented rand(-10..10) is also a nice thing to have.