No such thing as free food and drinks in Vegas, it���s an Oxy-Moron. As mentioned the easiest way is to sit at a video poker bar and plug one of the machines with bills plus a tip. The downside of this is none of the VP machines at the bar are full pay (AKA sucker machines).
The next best thing is a table game like BJ. But again you are playing a minimum of $3-to-$5 per hand plus a tip to get a drink. There a few places with a limited number of tables for 1 or 2 dollars per hand, but not on the strip.
On the strip, dinner comps are hard to come by. Most require $25+ per hand at 4-hours to get a coffee shop comp. Room comps on the strip start around $100 per hand @ 4-hours play. There are some places downtown where $5 or $10 per hand will get you dinner and room comps, like Gold Spike and El Cortez.
And the worst place is the low limit slot machines. These are the machines near the entrances far and removed from the serving stations, but once you finally get a drink here, if you tip really good the CW will check on you every 30-minutes or so you big spender:)