I'm gonna try really hard not to sound like a jerk here, but I sort of think it's inevitable. No matter how I say this, I'm gonna end up sounding mean and jerkish, so I guess I'd might as well accept that now and get right to things.
This is coming off a bit like someone getting assigned some difficult math homework, and then saying to themselves, "I can't figure out this problem. So I'll post the problem online and wait for someone else to solve it."
Which I'm normally fine with. Hell, everyone gets help sometimes.
But just like the 10th grader who has to go online in order to find solutions to a 9th grade level math problem, it's sort of the same thing. The difference...at least with homework, you're sort of FORCED to be there (until you're an adult, or you've graduated). But I sort of think that by the time one starts to take on voluntary photography assignments, is sort of the time by which someone should be able to come up with IDEAS for themselves.
It'd be one thing if this was a question about a technique or a method. As in, you had an IDEA for the shoot, but that required using a certain TECHNIQUE which you're not all that familiar with. In that case, sure. Go get help. Although realistically, you should have something else to fall back on if your technique is lacking due to your lack of experience with that technique.
But inability to come up with an IDEA is not the kind of thing which one should ever have to ask on this kind of public internet forum. I am strongly of the opinion that if one cannot come up with ideas on their own, then one should not be taking on voluntary assignments for clients.
The ability to research is an important skill, but I absolutely don't think that applies to "I have to do a shoot and I'm out of ideas. Any suggestions?"
My suggestion: live with the fact that you agreed to do the work, and then come up with your OWN idea without having to ask someone on the internet to give it to you.
And that is really for your benefit more than anyone else's. Firstly, a hell of a lot of the ideas that you get suggested to you are likely to either be crappy, or not really feasible, or good ideas that just don't fit with what the client wants. No one here knows the context better than you (since you're the only one directly involved with that client, that location, that sign spinner). Everyone here can give a buttload of suggestions, but you're still gonna have to weed through a bunch of suggestions that are garbage (garbage because the people giving you advice are dumbasses, garbage because the advice is good but doesn't work for your specific location/client/etc). And secondly, coming up with IDEAS is just sort of basic problem-solving. One should NOT take on assignments for others until one is capable of coming up with their own ideas. This can potntially save a person from taking on an assignment and then failing to deliver due to a lack of ideas.
So yeah...I know this probably makes me sound like a total jerk. But no one forced you to take on this assignment. I think the best thing to do is for you to cone up with your OWN ideas and not ask people to hand them to you. If you're at the point of accepting work without having an idea already in mind, then you're at the point where you don't have to go online and ask people to help you come up with ideas.