There is an old Celtic tradition which involves the humble snail.
Now the snail carries its house on its back and so it is never homeless; but more, the spiral design of the snail shell was always very special to the Celtic people, so it makes perfect sense that any kind of snail makes the perfect house warming present.
To be warm, safe, snug and protected inside one's own home is a wonderful idea and the snail symbol, or the more abstract spiral, lends itself to all manner of wonderful house warming gifts.
You can make a card very easily by cutting out a spiral and affixing it with glue; any snail motif on a greeting card, or a picture of a snail drawn or cut out from another source will make the house warming card special.
Snails are very popular as ornaments, so you can get an enormous variety of all sizes of snail figurines, from small brass snails to large garden ornaments made from marble and they're all easily found, online and off.
An exotic snail shell, or the shell of a sea snail is an extension of this idea and particularly good for beach front properties.
As the basic snail/spiral shape is natural and easy, you can even just find a beautiful stone and paint a spiral on it for a superb and extremely personal and lovely house warming present; further, it is the type of gift that the recipients will treasure and keep, and that will remind them of you and your good wishes every time they see it.
There are many other ways in which the snail design can be used for house warming gifts. If you really need to give a pot plant, you can decorate the pot in which it comes with a spiral. Any budding artist can make and paint a basic snail from self hardening clay; snail and spiral mobiles are easy and always fascinating, too. You can make a small painting that can be hung in a hallway and depending on the recipient, grandchildren can get involved, just the same as famous interior designers, artists and goldsmiths - it only depends on your budget.
I think that the idea of the Celtic house warming snail is a wonderful way to convey good wishes to someone in their new home, and to give them your intention that they should always be snug, happy, well cared for and safe.