Any improvements that suit your needs and lifestyle are worth the investment----just don't expect the person who eventually buys your home to pay for the cost of your projects. Before undertaking any improvement, be sure to bring routine maintenance up to date. It's throwing good money after bad to add air-conditioning or a deck if you've got peeling paint inside and out.
A home's value should also be a consideration. In general, the more expensive the home, the more likely it is that a Buyer will pay extra for improvements. Also, cost recovery figures should be adjusted for your neighborhood/location: understand what is most valued in your market: what is your climate? (is central-air an amenity or necessary?)
Some home improvements are a good investment regardless of climate: an interior facelift is the 2nd highest-rated remodeling project. It's a good idea to keep the inside of your home spruced up, since the effort could possibly net you appox. 109% return if you have someone else do it/appox. 210% if you buy the materials and do it yourself.
Eye appeal in bathrooms & kitchens are important @ resale time. The fixtures in "older" homes are usually colored and out of style.
Fireplace/garage/basement conversion/exterior paint-siding/deck/furnace replacements are either amenities or necessary and preventive maintenance/economic life of each could impact your resale value.
Source: The Home Book-Library of Congress Catalog