Dingle afternoon activities?

Have a pint at Foxy Johns which is hardware store.
Get ice cream at Murphys. Meander, take pictures.
Drive Slea Head. Visit Coumeenoole Beach. Walk out to the celtic cross on Dunmore Head. Pay respects at the Dun Chaoin cemetary and try to imagine what their lives were like. Take a Blasket Island Ferry if they are running.

If the Blasket Island ferry is running, take it out to the island for a day trip (leave in the morning, not in the afternoon, so you have the whole day). The ferry from Dunquin is amazing -- don't miss it.

Otherwise, amen to all the above. Especially to enjoying a pint at Foxy John's Hardware Store & Pub. Catch some trad music in a unique performance space. Don't miss the fish and chips at Reel Dingle Fish (arrive hungry!).

If you're looking for some easy exercise outside of town you could hike up the hill to the Eask Tower. Great views from up there, uncrowded, you hike up thru a farmers field to the tower. Side step the sheep. Need a car to get out there but it would be decent for a couple of hours. I would also consider just seeing if any music is available in town. We also attended a local gaelic football game. If one is going that afternoon it was a real hoot and a big slice of local flavor...those folks were awesome and the game was exciting. Probably a pretty slim chance of there being a match the afternoon you are there but if you enjoy live sports that's an option. Here is Eask Tower info
https://www.gpsmycity.com/attractions/eask-tower-5381.html

I was on this very tour in 2017, it’s a great one but I still wish I had signed up for the 14 day tour which also covers Northern Ireland. In any case, this is packed with exciting activities.

I looked at your tour itinerary and you will be taking the Slea Head drive and visiting the Blasket Island Center earlier on this tour day. Both of these are highlights!

In the afternoon and evening we wandered the streets and like the other commenters have said went into the pub they mentioned (I forgot the name). Also, go down to the port and find a bench and people watch. Totally charming town, I loved Dingle. My tour spent 3 nights there. RS changed it to 2 nights to add a night to Kilkenny.

For something a little different consider heading over to the village of Annascaul - about 15 minutes from Dingle town - where you'll find the Tom Crean South Pole Inn. Crean was an Antarctic explorer and was the 2nd officer on the Shackleton expedition back around 1914. He was a key figure in the crew's efforts to survive the long Antarctic winter after their vessel, the Endurance, was crushed by the ice. You can research his amazing career on Wikipedia, but the point here is that after he retired from the Royal Navy he returned to his home in Annascaul and opened this pub which he operated for almost 20 years. Don't know the subsequent history of the place, ie whether it's owned by his descendants or what, but I do know that it's one of the most convivial pubs in Ireland and is basically a mini-museum featuring photos and artifacts from his long career. It also happens to serve great pub grub and pours a terrific pint of Guinness ... and there's usually a warming peat fire going in the fireplace. We spent much of an afternoon there wandering around looking at all of the old photos and generally enjoying the ambience of the place. If memory serves there's a small monument in a little park across the street commemorating his amazing life.
If you have the interest give it a look.

Wander into the Dingle Music shop and see if there is a concert that night. If there is, buy tickets and GO!

If you have a car, the drive up Connor Pass is beautiful and offers great views of the ocean.

Since you will be on a Rick Steves tour you won't have a car and TBH, I wouldn't rent one for an afternoon.

When I was on the 14 day tour we did Rick's walk out along Dingle Bay starting at the Coast Guard Station. It was long enough ago that Fungy the dolphin was still inhabiting the bay and we saw him head out to the tour boats. The description on how to get to the start of the walk was in the Ireland guide book.