How to spruce it up? Begin by setting a realistic budget. What do you want to accomplish and how much money can you afford to spend? Once that is established, one simple solution is to paint or stain the walls and/or ceilings in a light color or pickled white. The rooms will immediately appear larger.
Next, I would look at the light situation in the house. Consider replacing the windows with larger more energy efficient low-E windows, like Andersen 400-series. Then focus your attention on the existing lighting fixtures and replace the outdated ones with a more current design. Nothing spruces up space in a timber frame like bold, large art pieces so remember to accentuate your art with proper lighting which will create bright strong focal points.
If you are planning to build an addition, I suggest you find a post and beam home company who has been building timber frame houses for a substantial period of time, like Yankee Barn Homes. They will help you design the addition so it complements the original house and fits your needs. Good luck and have fun!