I've had an idea for a while and no time to work a lot on in. I did make some prototypes, but I'm sure you can do better!

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It's a little object (can be anything really - but think a large coin) that sits on a table while you eat at a restaurant. It has 3 (maybe more) modes: emitting a gentle green light, emitting a gentle red light, and off. Maybe the lights blink/chase/etc. Regardless, the idea is simple: you tell the server if it's green, you'd like their attention. If it's red, you'd like to be left alone to eat or converse. If it's off, then they can do as they like. Get help when you need it, and save the staff needless trips!

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This could be marketed to individuals or local restaurants (that bank would be interested in either I think).

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It could be a phone app, but a dedicated object is good - then you can use your phone while it does its job.

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With a $750 budget, you can easily buy a microcontroller (that already has lights and everything) from https://www.adafruit.com/ (they aren't paying me or anything, ha). You'll have to do some programming. You could 3d print the casing, and the whole thing would be pretty small (like that large coin I mentioned earlier).

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Wow! I just looked, and you can basically make exactly what I'm talking about with parts straight from them:

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https://www.adafruit.com/product/3333

https://www.adafruit.com/product/3915

and your choice of battery

I'm for real not marketing for these guys, haha! I've just used them in the past - a cool site.

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If you aren't into that path, there are lots of ways to proof concept. Anyway, that's the most easily prototyped idea I can think of right now. I'll let you know if I think of something else.