Best way to give a surprise gift?

This is kind of confusing with the relationship, so bare with me! My husband's ex-wife gets a long with us pretty well and we share 50/50 custody of their 7 year old son. Her sister who lives out of state called me and asked for help for mothers day. She is buying a laptop for my husbands ex and is having it delivered here so I can wrap it and everything because she obviously can't do it. She wants it to be from my step-son to his mom....but I'm not sure how to have him give it to her. She asked I hide it in his room but I'm not sure how I would do that without her seeing me with it. My stepson and I were thinking of doing a scavenger hunt of clues to find in her house but again I can't do it without her seeing me. What's a creative way he could give it to her?