Yeah, a zip line aka “flying fox” (British term?)
The top mount of the ladder could be moved inward, and the anchor ropes moved further away from the playhouse, (to create a more ‘bridge-like’ ladder). This would enable access up from the ladder into the interior. If you want to make the ladder more difficult to climb, locate the lower two anchor ropes closer together
A ledge could be placed on the left hand side of the stair rail to walk onto after climbing the wall? But the lowering mechanism is useful - maybe there’s a little harness to wear and a simple pulley at the top? (a spotter is then required, or a counterweight). Maybe the zip line is at the top of the wall so it’s the ‘reward’ for climbing up?
A small trampoline sunk into the ground under the wall would be a dual purpose tramp+safe landing
Size wise, the carpenter will utilize wood panels and easy lengths of timber for efficiency, so you should make it as big as the materials practically allow. I couldn’t imagine a playhouse ever being “too big”. The play structures you see in local parks have very small interior spaces, because they don’t want kids hanging out there for long periods of time (or getting lost). At home you will have kids throwing ‘tea parties’ and doing ‘clubhouse’ activities that carry on for hours and involve numerous kids, rain or shine…
The small crossbar on the swing support could be lowered/widened to make it a height/size the kids can climb/dangle on (also, you can do your quota of daily chin-ups).
This could go on forever! I wish i got to design more playgrounds in my day job…!
You might enjoy this site for some ‘theme’ inspiration:
http://www.barbarabutler.com/onlinestore.php?mid=13§ion=kits (I love the water cannon idea)