Get the present this year - I'm sure you will think of something, and by next year the moment could have gone. (or you have other expenses). It is a lovely thought of your DH and kind of rude in a way not to take his lovely offer up.
I wouldn't not get a ring just because it's tradition where you are, but if you aren't keen anyway that's different. What about a Tiffany's necklace (if you like that sort of thing) there are many varieties not just the well-known designs, you could have a look on the website for ideas. It's a nice thought that in the dim and distant future you could be lending it to DD for a special occasion :) or pass it on to her.
Another idea is a charm bracelet, so you could add to it over the years. If Pandoras aren't your thing then you could get a traditional one. Links of London do some lovely plain ones as do Tiffany's.
If you would wear a watch then that's nice for every day but it might not last as long a piece of jewellery. I personally wouldn't go for a watch 400 euros because they tend to be fashion watches in that price bracket and won't necessarily be looking pristine/working correctly/still be in fashion in 10, 15 + years time.
Something simple like a gold twist bangle (if you wear gold) is unlikely to date, always look good and hold it's value well (nice to know but not essential of course for a sentimental gift).
I would browse jewellers stores and places like Links and Tiffany's online just to get an idea of what 400euros will purchase and get an idea for designs you like.
But please don't leave it until next year. Now is when you DH wants to treat you, let him have his pleasure from getting you a lovely reminder of what has been a very special time in both your lives :)