There are two main reasons people take on home improvement projects: for their own pleasure and in hopes of increasing the resale value of their homes. As long as you spend time and money for the first reason, you'll be okay. It's when you start thinking more about the second reason that you may be in for a disappointment. Even if improvements add value to your home, few recoup the money they cost. For example, a swimming pool, which costs tens of thousands of dollars to install, adds little to no resale value to a home. People are turned off by the maintenance aspect, and parents with small children will worry about safety issues.
So, what improvements do add enough value that you'll come close to making back what you spend? It's the highly visible improvements that will help most. Unfortunately, the less visible ones don't impress. Replacing the roof or furnace, while they may need doing, won't convince a home buyer to pay more. Likewise, new energy-efficient doors and windows are not likely to recoup the cost of installation (However, if you are going to continue living in your home for the next few years, $1,000 spent on insulation and caulking existing windows and doors can help you save on your energy bills).
The biggest home improvement you can do to impress homebuyers (and make them want to pay more) is one of the cheapest. Paint. As long as you pick colors that will appeal to a wide audience--and you do it right before you list (before it has time to get mucked up)--you'll increase appeal. Likewise crown molding is relatively inexpensive to install but adds the wow factor to otherwise regular rooms.
So, what else is worth doing? You may have heard that kitchens and bathrooms are what sell a house, and this is true, but don't think you have to spend tens of thousands on a whole-room remodel. Getting rid of outdated wallpaper or paint will help tremendously. Also sand and paint/stain old cabinet doors to make them look new. Switch out the dated cabinet hardware for something stylish and modern.
Any time you can take dead space (i.e. an attic or basement) and turn it into functional space (i.e. bedroom, office, media room), you stand a good chance of recouping much of the money you invest in the project (85% of the cost returned at resale is the national average).
If your house only has one bathroom, you can recoup about 90% of the cost by adding a second. (Adding a third gets you back less).
Landscaping can also improve curb appeal and salability of a home, but don't spend thousands on hiring a professional (it won't increase the value anywhere near that much). Tackle landscaping projects you can do yourself instead.
Keep this advice in mind before you open your pocketbook to hire contracts and buy materials for massive remodeling projects. If you're going to spend a lot, make sure it's--first and foremost--for you and your family, with resale of the house a distant second consideration.