A lot of great suggestions here...
One I would throw into the mix would be what I call the 100 in two.
I learned this drill from a DGA video I believe, I can't remember. Anyhow, the gist is that good players will get get their disc in the basket within two throws when 100 feet out.
So grab your basket (or find a tree), pace out 100 feet and try to get it so your approach leaves you with a nice easy putt.
Don't forget to vary your shots though!!! It's very important. You won't always have a clean line to the basket so try and find obstacles to throw around/through/over etc. If you don't have obstacles, pretend that they are there and work on a hyzer approach, an annhyzer, straight, low shots, flop shots (what I call going over a tree), etc...
You'll be surprised how quickly you begin to learn each of your approach discs. For instance, I use my putter on nearly anything within 100 feet, but will grab my roc for extreme hyzer shots.
Play around... you'll quickly see why this drill translates so well to the course. At the very least, you're getting both putting and approach practice in, you don't need a HUGE field, and it's something you will do on EVERY hole.
Hope this helps!