1) Does the restaurant have TVs? If so, remove them. Free.
2) Does the restaurant play music? Turn it down or off. Free. (Down is often better, because you want a baseline level of sound to cushion against sudden spikes like someone dropping a tray.)
3) Carpeting cushions noise and can be affordable.
4) Wood reflects less sound that harder materials like metal, brick, or concrete. Cost varies.
5) You don't need expensive acoustic panels. You can make your own. Buy a cheap roll of egg-crate foam and a stack of the cheapest art-framed canvases you can find. Either paint them yourself or hand them out to artists to decorate. This can be really fun. If you want to be unobtrusive, however, you can just paint them the same color as your walls. For a fancier look, there are places that make these things with art preprinted on the front. That costs more, so it's a budget question.
One of my favorite restaurants, Kohinoor in Champaign-Urbana, has a little vestibule with a trickling fountain in it, like a tiny quiet room you walk through. I love that. It's so peaceful and soothing. There's a couch just inside the restaurant where you can sit and relax while waiting for a takeout order. It's like sitting at your Indian friend's house. Sometimes they play Indian music in the background, but it's quieter than most American-style restaurants.