Hi ayse I have a retired friend who chooses short courses that don't have exams at the end, but are continually assessed. She's done five in the last couple of years and is now doing a science/genetics one, which she is loving. She certainly finds her courses challenging and absorbing, and the set books and guidance notes are fabulous. I always dip in them when I visit, and we share so much of what she is doing - last year when she did a botany-type course, we visited ancient woodlands, and were in my local park measuring ancient oak trees and identifying different mosses and lichens - fascinating!
I got my honours degree via the OU and loved every minute - it helped get me a career. Do you have a local tutor group you can join to go through your TMAs? You could invite them to you for an evening or afternoon once a month, or meet up at one of their houses, and they're really helpful, especially as you can email each other or talk on Facebook - there are so many OU undergraduates on Facebook, chatting about all sorts of issues to do with their specific courses.
The exams are the usual 3 hour sessions in a local uni, and you can take certain books into some exams. The last stages of each course are all about note-taking and revision and you have lots of time before the exam date to really go over your course again, but you don't have to do exams for many of the shorter courses now.