Scratch

I've never tried something quite like that and don't have access to Scratch at the moment, but try this:

when gf clicked
set [yvel v] to (0)
forever
change [yvel v] by (-0.2) // gravity
change y by (yvel)
if<touching [any solid objects v]?>
set [yvel v] to ((yvel)/(-2))
change y by (yvel)
if<touching [any solid objects v]?>
change y by (yvel)
end
if<<key [up arrow v] pressed?> and <(yvel) > (0)>>
set [yvel v] to (5) // jump
end
if<key [right arrow v] pressed?>
change x by (2)
if<touching [any climbable objects v]?>
change x by (-2)
change y by (2)
set [yvel v] to (0)
else
if<touching [any solid objects v]?>
change x by (-2)
end
end
if<key [left arrow v] pressed?>
change x by (-2)
if<touching [any climbable objects v]?>
change x by (2)
change y by (2)
set [yvel v] to (0)
else
if<touching [any solid objects v]?>
change x by (2)
end
end