Hiking

Go checkout Paradise (https://www.nps.gov/mora/planyourvisit/paradise.htm) first! Lots of trails directly adjacent to the parking area, ranging from quick and easy all the way to days long affairs.

Edited to add image of a view from a paradise trail, I took it 2 years ago:

Once you've seen and hiked up to the glacier from there, I would highly recommend a stop at Comet Falls! (https://www.wta.org/go-hiking/hikes/comet-falls) You can walk/sit under the falls and look up at the 100s of feet of water coming down at you, really fun!

Edit x2, here's a view from under the falls about a yearish ago:

Main 2 park areas are Paradise and Sunrise. Both worth a visit but can be very crowded on nice days (even weekdays). You need to register ahead of time to enter Rainier National Park, so make sure look into that. I would recommend a full day at each depending on how much time you would like to hike.

Definitely stop by Tipsoo Lake at the top of Chinook Pass. The drive alone is worth it. Also a great day hike area. (Very busy and popular)

Just for reference and depending on your hiking experience, the Wonderland trail is a fully connected trail system the wraps the entirety of Mt Rainier so if you are able to setup a thru hike of a section(car at both ends) it’s absolutely the best way to experience Rainier without the crowds. I highly recommend Mystic Lake to Sunrise. Be warned, 17.5 miles and just under 5000’ elevation gain. It’s a doozy but well worth the effort if you are capable.