Meat, if you have a big freezer

sundried tomatos.

Peanut butter. Seriously. The prices are about to skyrocket.

Olives, jams and jellies, canned tomatoes, dried beans, a variety of good jarred sauces for a quick weeknight meal.

Salad dressing, rice, canned veggies, frozen veggies, cereal, eggs, soda/beer/wine... In addition to the other ideas!

honey, sugar

I don't know about you but my supermarket has a kitchen essentials aisle with pans. They can sometimes be good quality...

I'd go for anything in the "I usually don't splurge on this sort of thing" category. I can get staples on sale all the time. I can not get an awesome jar of truffles or other specialty items on sale. Go for the imported stuff.

Stocking up only makes sense if it's something you'll use, so I would just get a lot of whatever I normally buy...probably plus a couple new things that are just fun to try while they're on sale (the splurge category you mention). But don't "stock up" on things you don't normally buy. There's probably a reason you don't normally buy them...

Hm, if you have time & like to do things like that you could buy the ingredients to make something like jam and spend a sunday making jam for the rest of the year.
Apart from this, I would just buy lots of everything that you usually buy anyway (if it keeps long, that is) and also splurge on a few selected items.

Pistachios! ...also capers and pine nuts are two pricey ingredients that I like to use.

I love the Rainbow sale, too! (BTW: Rainbow is a vegetarian store ... no meat, chicken or fish.) I usually stock up on nuts (which I freeze) and good parmesan (because it lasts a long while), imported things like farro and dried mushrooms, miso (another long lasting item), TP, pretty handmade cards for my card stash, dish soap and other cleaning items, wine (of course), vitamins. If there is one happening in the fall, I also think about holiday gifts and holiday baking/cooking. Chocolate, too!

I'll say the opposite of some other commenters: don't spend your money on staples, these regularly get on sale for more than 20 % off the reg price. Get luxury, pricey items, treats. :)

When is the sale? I can't find anything about it.

I'm with Christine - some things don't go on sale often/ ever and are good "investments" for your pantry. I have a secret stash of cash for that miraculous day when I find cheapo pine nuts (this has never happened), and they are probably the #1 thing I would go for to the point of angering other shoppers.

Invest in high-quality low-moisture cheese like Parmesan/ Romano and the similar lovely Greek cheeses none of which I can spell.

Buy saffron. Buy outrageous teas, if you like tea. Dried mushrooms and nuts are also excellent suggestions. Chocolate. Get some really awesome dutch-process cocoa.

And be sure to try something new!

Whole spices - they last a pretty long time if you keep them in a cool, dry, dark place (airtight containers are even better). And I second the saffron idea - it's so expensive!

I would also go for high-end specialty vinegars since they also last a long time if kept cool and dark. Ditto dried fruit and nuts, which can be hideously expensive in regular grocery stores (but not, thank god, at my favorite bulk co-op). You could also go for pricey specialty flours, especially if you've got room in your freezer for some of the more fragile ones.

And chocolate. You can't go wrong with super-high-quality chocolate. You will find a use for it.

Oooh, I'd join you at the sale if I was willing to brave the crowds, but being short in that store during a sale is a guaranteed recipe for an elbow in the face!
On to your point, I'd stock up on cocoa powder. I bought a bunch (those little baggies are deceptively big) and have managed to last through the high rise in cocoa prices so far. If you happen to have an in on what goods are going to skyrocket (like the peanut butter comment above), I'd go for those. If I had to last a season without peanut butter, cocoa, or butter because of high prices, life would be very sad indeed. And that's what freezers are for anyway!

I'd definitely consider buying six month's worth of butter and putting it in the freezer.

Plus what everybody else said.

Buy a decent stock of emergency supplies in case of a natural disaster or something. Seems wasteful to buy loads of other stuff if you may not use it in light of the new finding that Americans throw away 1/3 of all the food they buy.

I'd restock my freezer with the pricey stuff - meat, butter, and any prepared foods you might enjoy (Alexia oven fries, yum). Also, nuts, oils, vinegar, baking supplies, and rice/pasta.

I'll second sarahmitroff about the date of the sale-- do they announce it on their website? Do you need to be on a mailing list? I'd definitely make the trip across the bay for such an event.

And as for what to buy-- maybe use it as an excuse to splurge on a few pricier bits of produce, but I'd mostly go for non-perishables and things I know I use often. It's too easy to buy all sorts of things you'll never really use/don't even like all that much when the price goes down. Make a list with a little wiggle room, keep your head down and motor through.

You definitely want things that do not go on sale often.

Good Parmesan and other cheeses is a good choice, as are spices and wine.

Peanut butter is a good choice if you use alot, but the prices may have jumped by time the sale goes on which would make them still 20% higher than right now.

What items on your weekly shopping list do you often leave out or wince as you put them in the cart? butter, milk, olive oil?

Do you bake for the holidays? Stock up on sugar, flour, decorator sugar/sprinkles and food coloring as well as chocolate, nuts and extracts.

I love, love, love Rainbow. I fell in love while living in SF - now that I have moved to Oakland I still cross the bridge once a month to shop there :) My hands-down favorite item that I always stock up on there is the whole wheat gemelli pasta. There is nothing like it anywhere else. Also their 'Love Your Liver' tea blend. Yeah, when are those sales?

Rainbow Grocery does not announce the date of the sale until it's really close.