You can walk to Universal from DoubleTree. It is about 15min walk. I walked over twice last week when I was there. During the afternoon walk (3pm), I saw a LOT (50+) geckos on the pedestrian walkway on Major Blvd. I felt like I was the frog in the Frogger game. Every few steps you see a gecko run pass you. I only saw a few when I walk over in the early morning around 8:30am. My mom was a little freaked out. I made it a game on the way over to CityWalk.
DoubleTree do offer free shuttle to Universal. They utilize the SuperStar Shuttle Route A. But, boarding passes are limited. If your party is over 6 people, they don't allow you use their shuttle. So, if you do use them, get the passes the day before. Even if you don't get it, it's a short walk to Universal. And the return trip, you don't need a boarding pass. This route stops at Homeward, Hilton, DoubleTree and Four Points. They run 3 times in the morning and 3 times at night.
8:25AM is the first pick up at DoubleTree
5:20pm, 6:35pm, 7:20pm are the return time on this route.
Hope this helps you decide.
