I could use a little help. About five years ago I found this one in a second hand store, at a grand price of seven dollars or so. I paid for it, brought it home, put it in a wardrobe and promptly forgot about it. Now I have brought it out however, and one question nags me.... exactly what am I supposed to be doing with this contraption?

So, I saw this thread and thought I'd play catch up, because fitness is something that interests me. I've been out of the game for a few years, and I'm getting myself back into it. I gained 60lbs since I left the military and I'm looking to lose every single pound of it haha.
To answer your question, this is a decline bench. At face value, Nico is right, it is designed for sit ups and crunches. But it can also be used for decline dumbbell press, or bench press (assuming you have a rack to set the bar on). Those are two very simple and upfront workouts you can do. The decline dumbbell/benchpress works the lower parts of your chest, primarily. I've never looked into this particular bench, but I'm sure if you google it, there is a world of creative workouts you can do using it. You can use the angle for push ups if that is a weak area for you, but setting your feet at the bottom near where your head would typically lie, and place your hands somewhere along the bench (adjust your feet as necessary). The higher up your hands, the easier the push up, the further down you go, the harder they become. I'm sure there is plenty that you can use it for, but it is getting to be fairly late at night for me, and after my own work out, so I'm starting to hear my bed calling me.
One thing that I noticed, and maybe I missed something or just don't know, but are you doing the same workouts every day? That can pose a problem for your body. Abs is one thing, you still want to give them proper rest and a break, but you use those for just about everything in some way shape or form anyway. However, if you're working your arms daily, that can cause some issues. Your muscles need time to recover, usually about a 24-48hr period. That's why body builders and weight lifters have specific days, IE: Chest day, leg day, arm day, back day or Back/Bi's, Chest/Tris, Shoulders, Legs, Cardio, whatever their preference on how they break it up. It's to ensure they don't over work or over use the muscles.
I forget who said it, but signing up for something like Bodybuilder or Men's Fitness is a good way to learn more work outs, or even find specific routines to help with your daily workouts. The communities tend to be very helpful, and positive. I joined a website called Superhero Academy, and it was created by personal trainer who researched different celebrity/superhero workouts, and created routines from them. He puts in a lot of work to developing everything from dieting to specific types of training, such as body building, fighting, yoga, bodyweight exercising, running, etc...
To answer your original question. I swim, and weightlift primarily. Swimming is a wonderful workout, great cardio, and gives a good solid warm up to my muscles for when I go to lift. I have to lift lighter to be safer after I swim, but it creates a stronger muscle in my experience.