A thorough review of the documents setting up the development and the homeowner's association is in order before any meaningful advice can be given. Seek an attorney familiar with residential developments.
I agree that you are going to have to get an attorney familiar with your state real estate laws to answer this. There may be representation in the covenants, on recorded plats, in advertising, or elsewhere that can hold the developer to the original promises. If your development is large enough, you could even have federal land sales act implications that might help you. You can start with an attorney that is a member of the Utah Chapter of the Community Associations Institute: http://www.uccai.com/outside_home.asp
This answer is for general purposes only, and it does not create an attorney-client relationship.
I think there is a legal answer and a practical answer to this question. From a legal perspective, I think that the previous answers are good from a legal perspective. That is what do the contracts say, what did the developer's documents such as and plats show, what did the advertisements say, etc. Was there a pattern of statements by the sales staff and was that consistent or inconsistent with the contracts. I would suggest that you get the HOA to gather all of the documentation it can find from the beginning of the development to now on these issues. (Is the HOA still developer controlled? If so, you may have to gather that information independently of the HOA from the property owners.)
From a practical perspective, you should look into the proposal. If the HOA owned the park it would have to pay to maintain the amenities. So, without more information the issue of cost of maintenance does not seem to be an issue. The real question is the lease issue, what is the cost, term and restrictions of a proposed lease? If the cost is $1 per year, the term is 99 years, use is restricted to being a park and a HOA right to acquire for a nominal price should the developer ever want to sell then there may be no issue here at all. Why does the developer want to keep title to the "park?" Is there some tax, local code, financing or other reasons that are of benefit to the developer that do not adversely impact the HOA?
So, gather the information, discuss the lease proposal with the developer and determine the real impacts on the HOA. If there is a real issue then you will need competent local counsel.
This information is for discussion purposes only and does not constitute legal advice nor does it establish an attorney-client relationship. Individuals seeking a legal opinion or legal advice on any matter should contact an attorney experienced in the field of their issue and discuss retaining such attorney to represent the individual.