This is your sign that she and her husband can also pay for their own tickets, food, and transportation costs
Rest on your laurels a bit, OP. You can even hint that you would like them to treat YOU for a change. What I remember most from childhood roadtrips was my dad and his guidebook, stressing out about where to stay and what to visit. I really enjoyed taking my parents to disney last year and saying "I booked your hotel room here, go see this and this and I made dinner reservations already" My parents paid my way for so long to set me up to be in a good financial position...now it's my turn to return the favor.