Definitely lower level dining but a QC favorite is Rudy's Tacos. I like the one on W. Kimberly.
Get the enchilada plate combo. The smothered chicken is good as is the Cheese and Onion (artery clogging). I usually pick the beef for my third option.
100% NOT authentic, but it's the one dumb thing I miss when I'm in Florida for the season.
For pizza I would very very very strongly recommend checking out Smash Pizza in Bettendorf. Lopiez is also very good. If your wanting Quad Cities style pizza I always think that Bad Boyz does a pretty solid job.
I know other people will recommend it, but I would stay away from places like Rudy's, Happy Joes, and Hungry Hobo. All of these are kind of Quad Cities "cult classics" but end up being pretty mediocre to bad if you don't have nostalgia propping up your opinion of them.
If your venturing out a bit, Twin Span Brewing by the Bett Plex has really good food options and sold beer. In Bettendorf the Tangled Wood is also a cool option. I don't love their food, but they have a good drink selection and their patio area is worth checking out.
We used to be in the QC for a week each year. Tappas was a favorite, but apparently it is closed now. Was a good place shame. Sorry can't help any further.