Sounds like you're arriving on a US metal flight and departing on UA metal, which means you'll have to switch terminals. You can walk if it's nice, or take the shuttle bus. 2 hours isn't really enough time to leave the airport and go to a restaurant in Queens and make it back for your flight IMHO.

Once at the UA terminal, most of the restaurants are landside and they've been updated fairly recently. There's a food court on the lower level and a couple of decent bar/restaurants on the upper level. I can't specifically recommend any one over the others, but it's a decent selection. Also, the RCC is landside at LGA, in case that matters to you.

Security lines at the UA pier, IME, are usually <10 minutes. The beauty of having your lunch at the terminal is that you'll be able to keep an eye on the security lines and cut it as close as you want.