What should I buy for a house warming gift?

A furnace?

If they have been living together already, they probably have a lot of things. My thoughts are a bouquet of flowers and maybe a little bag with some kitchen gadgets you find useful (such as peelers or something like that) if they’re into cooking. If they’re not, maybe some nice soap in a gift bag with a bouquet of flowers. My thoughts go to flowers just because they’re something a lot of people don’t think to buy for themselves but yet are always an appreciated gift.

Cooking utensils and a cutting board.
A nice set of good non-stick pans.
And a nice refrigerator magnet!

If there is a good bakery in their neighbourhood, see if they offer gift certificates. Fresh baguettes and bread and muffins for a few weeks might be a treat.

Vase (glass) that goes with their decor.

I’m a big fan of not giving people clutter or dust-collector’s, so I’d be more inclined to go with a wine subscription or fruit basket subscription.

Depending on their hobbies/interests, you could always tap into that for something they could both enjoy, like a nice chess set, plant stands or beautiful Persian rug.

It’s hard to know without knowing them or seeing what they have/need.

People can usually use more or better kitchen gear. Upgrade one of their systems: new knives, enamelware pots, copper pans or mixing bowls, airbake cookie sheets, silicon baking mats, a better spice rack. Look at what they have, and what could use a little spiffing up.

A nice bottle of wine and some home-made cookies.

I kind of like @bob_ idea. Nice thought, and unusual.

A 30 years savings bond. I don’t know about other countries, but you might be able to get a $100, 30 year American savings bond. At 2.5 percent interest.

Also you can frame it for them.

A gift certificate to a nearby restaurant that they like for take-out, with the menu inside the card for quick reference.