Your bedroom is probably the most personal space in your house as it is somewhere that in general only you will go. This means that unlike the rest of the house you can give it a more personal touch. There are lots of ways that this can be done and many of them don't require a great deal of work. Of course you can also make major changes that involve major structural changes if you want to, the choice is yours. The most likely changes that you will make to your bedroom will be to change the colors. This can be done by both painting the walls and by changing the flooring.
In general it is best to stick with colors that are fairly light and neutral although you do have a little bit more flexibility than you would in say the living room. This is because the sense of size is not nearly as important in the bedroom. You will also likely want to get a carpet that is more plush for the bedroom than you would for the living room since it is more comfortable and since there is a lot less traffic it does not need to be as rugged. You can also change your bedroom decor and design by changing the window treatments as they have a pretty big impact on the look of the room.
Most homeowners will come to the point that once doing a handful of renovations, that they become "remodel hungry". Meaning, looking for other areas to renovate and remodel. Such as paingint and changing the carpet so that you can ensure a perfect color-combo match of your bedding and other bedroom accents.
If you are looking to really change the look of your bedroom and you are prepared to do some structural work you do have lots of options. One popular choice is to add a large walk in closet. This will of course require that you give up some of the space in the bedroom but if you have a large bedroom and are lacking closet space this can be a good option. This really only requires putting up some walls so the work involved is not all that major..
Another great option is to add a bathroom to your bedroom. The easiest way to do this is if you have a bathroom that neighbors your bedroom you can simply knock down the wall. The other option is to build a bathroom from scratch. This will of course require some major work as you will need to do all the plumbing and you will have to give up space in your bedroom, this works best if you have a walk in closet that you can convert. While there are some major hassles involved in adding a bathroom to your bedroom most people find that it is well worth it as it is one of the best improvements that you can make.