Been thinking about this and I would actually not install any new rides as a first priority. My first priority would be to go around every area of the park and paint, fix, and clean up any areas which look tatty, need replacing etc. Fix any broken themeing, (particularly in Duel) and just make the park not look so run down in places.
I would do the same in the hotels, refurb the rooms in Splash, fit fans into the rooms in Enchanted village, and look to improve the food offerings in all hotel restaurants.
As for the waterpark, fix all the water guns, effects etc not working (there are a lot).
Generally, a good clean up, repaint, maintenance etc would have such a positive effect on the park and hotels without spending millions on a new ride.