What can I use a 2007 iMac for?

The hard drives in those iMacs may be worn out or failing, especially after sitting for years. The power supply may also be bad from age. If, however, the iMac works and the hard drive is healthy, then installing Linux Mint on it is a good way to keep the computer useful for some tasks. This would require learning a new OS, but it is a great way to keep older computers alive since Linux only needs about 4GB+ although I have been successful using Linux on an older Mac with just 2GB of RAM with several browser windows/tabs open. However, Linux is not for everyone. You can test drive Linux Mint by choosing the "Live" option from the USB installer although it will be extremely slow running from a USB stick, but it will give you an idea of what it looks like & to confirm it works. It can also give you a chance to check the health of the internal hard drive by using GSmartControl which may already be installed or it may be possible to "install" it into memory while running in "Live" mode from the USB stick.

Many popular third party browsers are supported (Firefox, Google Chrome, Vivaldi, etc.), plus it has access to LibreOffice as well as lots of free open source software including video editing apps although I don't know how they will perform on such an old system especially since the iMac probably does not have much memory. There are also some open source games available as well.

I mentioned Linux Mint specifically since I know it will install & run on older Macs and its desktop environment options don't use as much memory as some others. Other Linux distributions may work or can be made to work, but some of them utilize a desktop environment that is too resource intensive for older systems (Ubuntu's default is one of those needing a newer higher end system to run).

Before erasing the internal macOS boot drive, I highly recommend you create a bootable macOS 10.11 El Capitan USB installer while you can so you can always go back to macOS if you decide Linux is not for you on this iMac.

Create a bootable installer - Apple Support

The best way to enjoy classic 32-bit intel mac applications (games, photo editing software, etc) that won't run on Big Sur is to just install High Sierra on an external SSD and boot into that on your current 2013 MBP. Get an external monitor if you want a bigger screen.

If you are still trying to put a worthless obsolete Core2Duo to some use...

1. Retro Computing as a hobby. Some run offline applications for nostalgia. Others try to keep the system online for the masochistic challenge. Snow Leopard for PPC apps via Rosetta or Mountain Lion for early Intel applications. (Anything that requires 10.9+ will run better on your MBP)
2. Raw material for an art project. Use your imagination. Frame the logic board on your wall. Gut everything between the glass and case and turn it into an ant farm. Etc.

Kudos for wanting to keep an old Mac in service! I try to do that as well. However, I have a couple of questions I ask myself when wanting to put and old Mac back in harness:

• Have you been able to successfully boot the computer and confirm it works normally? That is Step One because old Macs that have been sitting unused and off wall power for extended periods are too often non-runners.
• Are the apps you wish to use already installed on the 2007 iMac? I ask because it is nearly impossible to find versions of apps in stock today that run on old OS versions. Yours maxes out at macOS 10.11 El Capitan.