Distance from the hostel
Marni BKK Hostel lists the Grand Palace at 2.4 km. That is a longer walk or a short taxi or tuk-tuk ride, not the same trip as Khaosan Road at 500 m. This page does not publish a fare or a journey time, because Bangkok traffic changes both.
The palace is the historic royal complex in Phra Nakhon. Wat Phra Kaew, which houses the Emerald Buddha, is in the same grounds. Sanam Luang (1.7 km) and the Bangkok City Pillar (1.9 km) are in the same part of the old city.
Before you go
Tickets, opening hours, and which buildings are open change. Confirm them on an official source the day you visit. The palace has a long-standing dress code that covers shoulders and knees; check the current rule rather than assuming an older one still applies.
The Bangkok National Museum is a natural pair with the palace if you want more than the grounds. It is 1.7 km from the hostel on the same distance list.
Getting back
Ask the driver for Marni BKK Hostel on Soi Worapong, Wisutkasat Road, or for Khaosan Road and walk the last 500 m. If you are checking out the same day, leave bags in luggage storage and collect them after the palace. See the rooms if you are still choosing a bed.
Stay in Phra Nakhon
Marni BKK Hostel is the base these notes are written from. Check dorms and private rooms for your dates.