The Ninfeo della Pioggia (Rain Nymphaeum) on the Palatine Hill in Rome will reopen November 1st. It's been closed for decades.. Visitors will now have access to its 16th-century frescoes and ancient sculptures.
https://www.wantedinrome.com/news/italy-rome-restored-ninfeo-della-pioggia-reopens.html
The article also mentions the recent reopening of the Domus Tiberiana. A few more details can be found in this second article:
https://www.wantedinrome.com/news/italy-rome-domus-tiberiana-palatine-roman-forum.html
I imagine tickets are available from the Colosseum website, but it's not active right now so I cannot confirm that.