It depends on the type of hardwood floors you install and the quality of the installation. Real hardwood is the best, followed by hardwood veneer, and finally laminate which is equal to carpet from an appraiser's standpoint.
If the job is done right, with real hardwood, you should be able to recover most of the cost when you decide to sell. You also increase the desirability of your home in the eyes of potential buyers, which makes the house stand out, sell faster and sell for more money than your competition.
Also, coombined with other renovations such as a new roof, new heating and air, new cabinets etc., the new hardwood floors change the effective age of the home. You can have a home with an actual age of 20 years, but if everything is new or upgraded in the home the appraiser might give it an effective age of 5 years. An appraiser will make an age adjustment for an older home of around 1% of the sales price x the age difference between the comparable sales in the area and the subject property. For example a 20 year old home valued at $200,000 that has an effective age of 5 years this would be about $30,000 (1% of $200,000=$2,000 x $15,000= $30,000).
Adding a back patio would give you an extra $1,000 in real value from an appraiser if there was no patio before. If you currently have a patio that you are going to cover it will not necessarily add and real value to an appraisal, but like the hardwood floors, it will help to sell the home quicker and for more money. The appraiser has some wiggle room for adding value to a home based on the quality, or how they "feel" about the value of the home. And again, it depends on the quaility of the materials and workmanship.
Appraisals aren't an exact science. It is about 80% facts and 20% gut feeling about the value of the home. If the appraiser can see there is a willing buyer that has contracted to purchase a home from a seller, the appraiser is looking to see if the contract price is within the range of comparable homes in the immediate area. If the numbers are off a little they can add value to the home based on the quality of the home.
The best thing you can do to add value to your home is to just take care of it.