If you want some simple objects to work on to improve you skill with, you might try writing versions of some common basic utilities, such as
cat(1),
more(1),
wc(1), etc., or any other tool that doesn't use any system-specific operations. A quick look through the
man(1) page index and/or a list of MS-DOS commands should give you quite a few things to try; this is a fairly good way to practice in any language, actually. I wouldn't expect to outdo the existing versions, but you should be able to write decent versions of several of them fairly easily.
Also, try all the classic 'example' programs: I assume you've already done a
'Hello, World!', but there are always the
'99 Bottles of Beer' program, or a
ROT13 translator (see the
ROT13 polyglot page for examples). In some other languages, I might suggest writing a
quine (a program that prints out its own source code), but in assembly that would probably be a rather frustrating task. I might add that I'm thinking of setting up a polyglot page of my own, with a hex dump as the target; others I think would be useful would be a
dc(1) style RPN calculator, and a simple calendar-printing program, all of which are also good practice programs to try.
If your feeling ambitious, a simple line editor might be interesting - though that's hardly trivial in any langauge. If your
really ambitious, you could write a sort of mini-disassembler, but that's probably more than you want to tackle any time soon.
Or you could pop on over to the OS dev forum and try your hand at writing a boot loader
Good luck.