You don't say how far you want to ride.

I suggest you replace the drop handlebars with mountain bike bars and brake levers. I have done that with an older road bike, leaving everything else as it was, except removing the front derailleur. It's a nice ride, but I wouldn't want to ride too far since I ride a road bike for distance.

The big problem with doing that is whether the straight handlebars fit the handlebar stem or whether a different handlebar stem will fit in the fork tube.

If you want to buy a Goodwill mountain bike, you're pretty safe that the brake levers will fit. You might go to a local bike shop and find a good, inexpensive straight, flat handlebar.