Any books you can think of that would mesh with any special interests...a beautiful coffee table type book - travel? sailing? mountains? world beers (don't all Germans have to love beer?

A beautiful bowl or platter from an artist near you or that you love?

Along with a gift certificate to a nice restaurant in their area?

Wow, how lucky to have such a wonderful and giving relationship not only with your daughter's boyfriend but his family too. Maybe you could send a small art piece from a local artisan for their home. I know art is typically in the eye of the beholder, but maybe you can find a few pieces you like and then send pictures to your daughter for her insight as to what would work best in their home.

A gift card to a nice deli nearby-to-them where they might select some special cheeses, salami, wursts, and spreads that they might not otherwise treat themselves. Germans LOVE their preserved meats and cheeses as a light supper.

Certificate for tickets to a Berlin concert or play (again a treat they might not otherwise have).

A gift card to the huge, I mean huge! KaDeWe department store where they could select from everything in the store (including a big top floor of high end foods, as well as a restaurant).

If you need to bring your gift with you from the States, then I'd suggest some pairs of Levi's (if your daughter can get the sizes), or electronics (like an iPad mini depending on your budget of course!!). Both those things are so much more expensive in Germany that they are often bought here by visiting Germans.

If the budget is much smaller, then another item they can't get there but like to have is bags of chocolate chips for baking. (It's so weird that they have to pay a monster price at a specialty shop to get the imported US ones!) You could bring a cookie plate themed from your home area, and some bags of chips.

Weekend holidays are big as a way to get away from the city. There is a region of Bed and Breakfast type accommodations in the forested area north of Berlin, and you might give them a nice weekend getaway for the upcoming Spring.

You probably already know this from previous experience, but the gift wrapping is also extremely important, even for a gift card or certificate, and your special care with that will definitely be noticed and appreciated!

Many Germans are very into wellness - mineral bath spa type places are very popular there. Google Kurort, Heilbad, or Therme to find various options. If the parents enjoy this type of activity, a gift certificate to a place like this might be welcome, either for a day trip or a weekend/longer stay.

I think you have to tred carefully here. The problem with gift certificates of any kind is that they have an exact monetary value. And since their son is staying at your house for an extended time, they may feel the need to reciprocate. Maybe you could host the dinner in Berlin and thank them graciously with a toast during dinner?

A gift from California that is practical, functional and will be used. Consider sending extra cash to you DD so she can treat her BF to lunches/dinners out to lift some of the cost of meals from the family. We had two different host situations involving families overseas. In one case we sent a gift from our native state, and in the other case we brought gifts from our area for each family member. Your situation is a bit more delicate, as you want to offer support but not offend.

I was going to make the suggestion Bonnie Ann did. Leave your daughter with cash so she can take the family out for meals or stop at the bakery or market and bring treats "home" with her.

Me three. A nice way to help out, yet quietly.

The Germans I know LOVE the American west. Perhaps a book or piece of art, or something Disney related. Flowers are a must to bring home at some point in your visit. Ask your daughter what their interests are (gardening? Dogs? Cooking? Movies? Walking?) and bring them books, art, or other items, related to the California way of the interest. And leave cash with your daughter to take them out.