Hidden gems are pretty much nonexistant - your best best is to find off season time to visit those mainstream parks. Avoid weekends school breaks holidays and you are likely to have a lot of space to yourself... moab during utah school break last month was packed but a day later on Monday it was a small fraction of that crowd.
If have to go on weekends go a week prior or a week after holiday weekend. Winter can be amazing and devoid of crowds - snow hiking or xcountry skiing in bryce are very scenic.