Chitwan National Park is probably the best known park in Nepal and is listed as a World Heritage Site - it's all in the ownership of the government and covers about 100,000 hectares on the border with India.
The main reason for Chitwan's status as a haven for wildlife is that it's one of the few places in Nepal that isn't mountainous - actually it's almost totally flat, with much of the area around the Gaida Camp being an extremely large flood plain (miles wide) - just before our visit the monsoon had caused the river levels to rise by over four metres - big rises are expected and housing is built with this in mind - either on stilts, on high ground, or from easily replaced materials such as mud brick. The main river is the Narayani which for our time there was a fairly shallow but wide river - the deepest point we became aware of was probably only two metres deep but this recent flooding had been more sustained and higher than usual so much of the ground had been covered with a layer of mud - this is good and bad.
The mud is brought from upstream and is very fertile so helps it the forest to regenerate, unfortunately by covering everything it reduces the food supply for the animals in the short term and had the effect of altering our plans for moving around the area as jeeps were having trouble negotiating the deep unstable mud which existed in some places.
We still had a great time and there were plenty of animals to be seen, the mud sometimes making it better as it was easier to identify the animal tracks. Still, it's probably best to leave a fair bit of time between the end of the Monsoon and coming into the park - one of the best times being at the start of November.
Things to bring - you should certainly be wary of your clothing, long sleeves and long trousers are an advantage when riding the elephants as otherwise you could get scratched a bit by tree branches as the elephants plough their way through the forest. Darker plain colours are also best in order to avoid scaring off animals or attracting unwanted attention - so avoid reds, yellows and whites, also in the same vein of thought you should avoid strong perfumes as this is nothing short of heaven for the mosquitoes.
Lastly a checklist of equipment should include, a torch to see around camp at night, cash - as it's often difficult to exchange cash when you need it most, a padlock for your valuables and decent footwear for walking safaris