Guide
Open a Reaper Project in Cakewalk
Cakewalk (formerly SONAR) can open an OMF file, which is what the converter makes from your Reaper project.
Go to File > Open, choose OMF File in the Files of Type field and select the .omf.
OMF doesn't carry tempo, so if you know your project's tempo, enter it in the Initial Tempo field when you open the file — that avoids clips moving later.
You can convert your Reaper project to OMF for free here.
Going the other way? Here's how to open an OMF file in Reaper.