What can I bring back?

Here's a link to Customs and Border protection explanation for your question.

https://help.cbp.gov/cgi-bin/customs.cfg/php/enduser/std_adp.php?p_faqid=82

Specific to your items:

While there is some concerns with soil and organic items, as long as the sand is dry and the seashells are clean and dry, you should have no issues with that. The article linked by another poster will point out that you cannot bring back any live plants, fruit, and any seeds would need to be inspected.

As for food, Chocolate is not a problem, and the dried herbs are not an issue either. Yes, do not bring back any meat items, but just about any other canned, dried, or packaged food item is OK. Cheese maybe is the only other "iffy" item, hard cheeses, packaged are OK, the smelly, soft, gooey cheeses are not. Any liquids/pastes should go in your checked luggage.

Customs is not a big deal, so relax. On the plane you will get a Customs Declaration Card, fill it out per the instructions, you will need to list a value for each of the items you purchased in Europe and bringing back, it need not be extremely accurate, unless you spent unless you have a couple thousand in items. There is a question about food items, you will need to check "yes", you may be asked what type of items, answer honestly, and randomly the USDA may pull you aside to check the items and your bag (happens to me about half the time).

Have a great trip home.

Whatever you do always write EXACTLY what foods you have. I made the mistake the first time and wrote "food" on the declarations card and got questioned. They didnt search me or my luggage but I think it was because it was late at night and the lady looked dead tired and waved me through after stamping my passport. But I suppose she could have searched.

The second trip back to the US I put "Chocolate, Cookies" etc.

I dont think any of those things will be a problem. But I wouldn't claim the sand on the customs form.

I've brought back a lot of goodies from France before and never had a problem. But I'm naughty when it comes to claiming things.

Please claim everything on your form. Anything that isn't allowed is because there is a legitimate health or agriculture risk to our country. You don't want to be the one bringing a pest, disease or harmful substance into the country. List the things you have, let them inspect if they want to. If they don't allow you to bring it in, it will only be due to a potential for harm.

I think sand that is dry and clear of bugs or plant debris will be fine. Sea shells that are fully dried out should be fine also. Anything packaged except meats is generally fine. For cheese, I thought pasteurization (which isn't normal in Europe) was the issue but it seems to be more about hard vs. soft cheeses as the earlier post mentioned.

I know for a fact that you can not bring back any packaged meats, even if it is a very nice 40 euro pepper salami bought in a duty free store! The cheese was no problem for some reason ...