My sister refused to use any bathroom but the ones at home!
With our daughter, we tried "bribery" with bikes and things, but at that age we just weren't sure she really comprehended what the "bonus" was all about.
What we ended up doing was this: no matter what she starts asking for: Potty first. If she wants a drink, to watch a video, or to start coloring in the coloring book? Everything is "You can, but go potty first." Or set the timer for 30 minutes, and every time it goes off....time to go potty.
Kids do NOT want to take the time to go potty. ANYTHING is more fun than going potty. Who wants to take time out from doing something fun to go potty? 3 year olds don't, and 30 year olds don't....how often have you sat through a movie when you really had to go, but you didn't want to miss anything?
Further, since they're young, they don't fully comprehend the "reward system". SO, instead of us coming up with a "suitable reward", we made whatever it is they want to do become the reward. They know they want a drink, it's immediately on their mind, and they really want it. "OKay, but go potty first..." and they'll actually do it!
Our oldest daughter would sometimes have "relapses" even at 4+, and we'd start setting the timer. When it would buzzz, she'd say, "I know....time to go potty." We'd let her know that if she would use the potty like a big girl, we could stop using the timer. And that was usually all it took.
For what it's worth, and good luck!