Awesome news and I would like to congratulate you.

Simple: A light picnic in the park: a nice bottle of wine (as it is light, I'd stick with something like a Riesling), some light treats/snacks, flowers (not roses) and maybe a little music (not the radio).

A bit more/Time away from the city life: pick a destination that is close to the mountains; acquire a nice suite (I'd go for the honeymooners). After breakfast go for a walk or hike (site seeing), then if you cannot find a lake or similar, treat him to a day at the spa or hot springs. Finish off with a light meal.