This app was mentioned in 5 comments, with an average of 1.60 upvotes
..Available in the Google Play store now, as I've just discovered! (Haven't tried it yet, but reviews look positive.)
I spent money on some ports from Beloko Games. Namely Doom Touch (which used to be on Play Store before he got permabanned). That was pretty much the first AND absolute best purchase I've made...especially with its GZDoom support (which has a LOT of mods from Brutal Doom to Demonsteele), and with third party tools like OBHack (a random level generator) made it FAR more worth my money than anything else....I bought it for 2-3 dollars.
He should still be selling it on Amazon though.
Sumotori Dreams - Basically some weird old physics sandbox game...its good for a few laughs or as a fun distraction while you're waiting for a movie to start, or on a train, or while you're waiting for a PC game's loading screen...also has really quick loading times so its pick up and play...and probably fall over seconds later.
Interstellar Pilot - A Space Exploration game in the vein of Galaxy on Fire 2 or Elite. I'm sure the scale is ultimately smaller than one of those 2 games, but its also one of the closest to have simple gameplay AND a good immersive sandbox experience.
Soul Unleashed - Not sure if you guys think its worth the money or not; but i had a lot of fun with this one. Its basically a turn based RPG with LOADS of character customization from a wide variety of skills (you only get to pick 3 out of a WIDE list of them unfortunately). Making builds is fun, the world while not open is immersive enough and you can interact with the world uniquely with the skills you picked.
One of my favorite mobile games!