Mac King for an afternoon show that is funny and clean, at Harrahs. Then she could gamble a little in the Harrah's casino.
If Rita Rudner is still performing, your g'ma would enjoy her one woman monologue...very funny, clean, and Lily speaks clearly enough to be followed easily. Not an assault on the ears.
Legends in Concert and Jubilee are vintage Vegas shows. She may have seen them already.
There is a similar show at one of the theaters at Planet Hollywood, I forget which one, perhaps the V?, but a small theater and musical 'history' of Las Vegas. BEWARE: It is a long trek to those theaters at Planet Hollywood. So you would want to combine that with a little gambling in Planet Hollywood.
Bellagio has good self-parking which takes you right into the Conservatory. Combine this with a little gambling there. Consider the Petrossian Bar for relaxing and taking tea or a cocktail, people watching.
I agree that unless she is unusually vigorous for her age, she won't be up to bouncing from place to place hours and hours at a time. But, from Bellagio you/she can walk down the sidewalk to the Cosmo. You could take the tram from Bellagio to Aria/City Center or Monte Carlo, but beware that it is a trek to get to the Bellagio end of the tram.
I recommend a breakfast at Mon Ami Gabi at Paris. Sitting on the patio is nice, but the "inside" room just inside of the patio is just as good if there is a wait for the patio. My favorite breakfast is Blueberry French Toast and a side of their excellent thick bacon. You could combine this with some gambling in the Paris casino.
So much to do...so little time! Hope you are staying for several days.
Sorry that I am not so familiar with Downtown. I'm a Strip gal.
Edited to correct female performer was Rita Rudner, not Lily Tomlin. Rita was super.