Furniture - invest in good furniture you will love forever... or on a budget, try second hand stores or hit garage sales / estate sales. IKEA has just opened this year here in Las Vegas. We also have second hand consignment stores such as Colleens Classic Consignment.
Lighting - place floor lighting or change out lighting where you can, you can swap back the original light fixtures when your leave.
Wallpaper - temporary wall paper is available. Instead of painting which many owners will not allow, this is a great temporary solution.
Art or Picture collage - hanging wall art or a picture collage, this allows you to put your personal touch into the home.
Rug - a nice area rug can do wonders to cover up dated flooring or carpet.
Curtains / Window treatments - Walmart or Target has a great selection of window treatments at a fantastic price.
Plants / Flowers / Herb Garden - this will bring a nice aroma to your home, and who doesn't love fresh herbs when cooking.
Paint - paint furniture instead of walls to give a bold fresh vibe
Work with what the properties aesthetic and not against it can really help your home feel more inviting and homey.