Sometimes it isn't about doing the whole wine and dine kind of night. Sometimes it's nice to go back to where it all started. Revisit the place you went to on your first date.
Try writing a love letter to one another, put thought and creativity into it. Then share it with one another over dinner.
Maybe you don't have the money for an actual spa, but you could try staying home and massaging one another, trying at home DIY facials, etc.
If you're in the mood for a little adrenaline, try going sky diving or cliff jumping
Maybe take a road trip that's spontaneous!
You can stay in to read, binge on a show, watch a new movie, etc.
A more creative list of ideas for a stay-at-home kind of night