The girl I like wants to travel alone. How can I go with her?

Know the city she is going to, help her plan her trip, book a hotel, recommend local customs, local snacks, local attractions, and explain matters needing attention. You'd better prepare her accompanying article. In short, the most intimate service is done in advance. If you want to surprise her, you can sneak in early and surprise her at the destination. But it will be more expensive. But this surprise will make her love you. Bless you.