I understand your concerns, especially this soon after your surgery...
- Completely avoid any carbonated drinks and any food items that you know give you gas.
- Take along enough toilet paper from home to be used for a couple of empties, just in case they are out or use the really cheap thin stuff.
- Eat a small meal and small desert to minimize your overall risks, just in case.
- I frequently use a muffler system when I go out. Basically, I use two neopreme oval shaped "mufflers" (sold by stealthbelt.com) over my bag. They have a lot of interesting products for ostomates. I twist the bag sideways, and pull a soft stretch wrap over the area to hold things in place. It makes it more difficult to judge how full the bag is, but I have gotten used to it over time. It also reduces how much the bag will hold, since it's twisted sideways. But it really reduces the sound noise, albeit with a little more attention required.
Have a great time!