I purchased the Garmin eTrex Legend a year or so ago to fish off the coast of Louisiana in the Gulf. I found it affordable and amazing for the water, and also driving. It has really saved me a few times.
I can't say how well this works in the parks, I didn't try it - but I will next trip to Inks Lake. For about $130, it has a great road map, is zoomable, lots of functions and features and easy to use.
http://www.garmin.com/products/etrexLegend/
Jamie