Any tips to watch out for when setting one of my databases to Read Only?
is it simply just changing the Database Read Only State to "True"?


yes, you can use below t-sql

alter database db_name
set read_only with rollback immediate



  Also, is it a good idea to change the recovery model for this old production database to "Simple" vs "Full"? 


Yes, a read-only database wont be having any DML operations. So best practice is to change the recovery model to SIMPLE. Make sure that you take regular FULL backups (and most important is testing your restore) - just incase if the server goes down.


  In worse cases, I always can set it back to Read and Write at anytime with no issues correct? 


Yes, you can just run below tsql

ALTER DATABASE db_name SET READ_WRITE
with rollback immediate
GO