One of the easiest - and pretty inexpensive - things Sid and I have done in our house is to install crown moulding.
From our front door, you walk in to our rectangular living/dining room. It is literally a long, long, long boxy room (and no foyer to speak of.)
Although we own a cute little yellow miter box, we decided to add some architectural interest - and miter-free moulding with the addition of wooden ornaments.
Check this out:
We bought wooden corner blocks, which are about $6 each at HD. To break up the length of the room, we also bought some half blocks, which we centered along the walls. You can see one of them here.
This is one of the corner blocks at the dining end of the room. I like the extra oomph it gives the room!
Another shot of the half block centered along the front wall.
This is how we solved the end of the wall meets the stairwell problem. Our front wall ends, and there is a short low ceiling (the bedroom closet upstairs fits here) before the ceiling opens up to the stairs. We used another half block to end the moulding in a way that makes sense.
This is how we handled the many ins and outs of our kitchen moulding. We had several interior corners and exterior corners. FYI, the outer corner pieces are way more expensive (close to $20 each) than the inner corners but made the kitchen look so much nicer!
How to do it:
1) Screw in the corner pieces near the top of the wood blocks. The moulding will hide the screws.
2) Measure and cut the moulding. In our LOOOOONG front room, we used very long pieces. Smaller spaces or more corners require more cuts.
3) Use Liquid Nails adhesive behind the edges of the moulding, at the top where it meets the ceiling, and at the bottom where it meets the wall.
4) Use just enough finishing nails to hold it up while the adhesive dries. Countersink the finishing nails.
5) CAULK! CAULK! CAULK! CAULK! CAULK! CAULK! - Caulk the any gaps between the moulding and the ceiling; the moulding and the wall; the moulding and the corner pieces; the corner pieces and the ceiling; and the corner pieces and the wall.
6) Let the caulk dry. Look for any additional gaps in any of the above places. If you find any, CAULK AGAIN!
7) Paint the moulding. I love Behr Ultra Pure White.
We also installed bead board wainscoting. More on that later!
(In case you're wondering, my front room color is Behr's Warm Terracotta. My stairwell is Behr's Arabian Sands. The kitchen and den are Behr's Whisper Yellow.)