Is it okay to say 我下一下 when e.g. getting off a bus?
I think it's ("60%" OK, "40%" wired ) to say "我下一下" to driver when you are getting off from a bus. Chinese people know what you mean, but few people would say like this. 

In most cases, people would say "您好，我想下车".  it's quit nature. 

your examples of "我坐一下", "我百度一下",  that's very OK, here "一下" means "try once"