Darker lilypad textures
Lilypad models (with both alive & dead versions)
Stuff to make the water more interesting - e.g. driftwood, underwater plants, fish particles
A couple more tree models
Convert some of the existing overlays into a sign prop (such as the intel one - but with white wood)
White, dirty wood texture
Many more algae and stain textures
Some sort of holiday park sign, like in L4D's Blood Harvest e.g. "Welcome to Gulliver Park"
A dedicated rolling mist particle
A seperate mosquito-only particle (assuming one doesn't exist already)
A picnic blanket texture (with plate and picnic hamper prop)
A skeleton ragdoll (I'd suggest modifying the one from Episode Two)
A lot of suggestions, most seem simple