North vs South in the Denver area isn't going to make any significant difference in temps, and would be hit or miss in regards to snowstorms.
It is true that some ski areas (A-Basin) might stay open until July 4th, but that means one run. If you want to ski, earlier in March will be better.
Cherry Creek and Chatfield are your best options for State Parks close to Cap Hill area. Others mentioned the 14 day limit, which is enforced. I also know that if you make reservations under different email addresses, and put your name down on one, and spouse on the other, that you can get around those limitations for reservations. I don't know if the Rangers would give you grief because that time of year, doing that would be obvious because there won't be as many campers.