If you want to spend a few hours or a day exploring below the rims, you can hike partway into the canyon on a number of popular trails, or travel by mule from the North Rim.
But if your goal is to reach the Colorado River, take note: The National Park Service strongly discourages anyone from attempting to hike from rim to river and back in a day. Unless you are an exceptionally strong hiker, you’ll need to spend at least one night in the canyon, and that means preparing ahead.
Mule tours from the South Rim  generally spend one night in the canyon, but you can arrange for a second night. A simple backpacking trip  might require three days and two nights in the canyon, but you’ll need several days if you take wilderness trails  from rim to river, and 1-3 weeks if you plan to explore the length of the inner canyon by raft .
An overnight canyon trip isn’t something to do on impulse: Careful planning and preparation are essential. You may need weeks or months of lead time to apply for backcountry permit  or to make tour reservations. You may also need time to improve your personal fitness.
Because of the myriad challenges (heat, wilderness, remoteness, topography), the inner canyon is an area best experienced with companions. If you haven’t been to Grand Canyon before, or if you and your companions don’t have a lot of hiking experience, it’s a good idea to sign on to a guided tour for your first inner canyon adventure.