Spend time compiling a scrapbook with memories and objects that remind you of them. Or go through all the things you own for objects they gave you. Fix up any that need it and use them for decor. I have a wool lap blanket my father brought back from Guatemala, that I cuddle with, especially when I'm missing him. Or make a collage of photos of them and the two of you together for a wall display. Or pick up a hobby that you both enjoyed and teach yourself new skills, imagining them there. My grandfather used to make jewelry. He also was a photographer for awhile, and I do both of those. With these few ideas as examples, I'm sure you can come up with something that's special to you.