My car is not clean. For some reason it is always the very last thing I clean, BUT I have discovered that it is something that can be cleaned fairly quickly. It is VERY satisfying to have a clean car.
If you cannot decide where to begin if you are starting to declutter your home, your car might be the pace to start. You vacuum and wipe everything down. You throw out trash and you tidy up the glove-box, the door cubbies, the trunk (if you have one) and the console. And if there are toys then just keep a box outside a door and toss the toys in as you go. Frankly the hardest part is finding a cord extension for the vacuum cleaner.
You can wash your car too. That is always fun on a hot sunny day. 🙂 Try to remember to get a little wet yourself.