Your Toyota Highlander has a listed towing capacity of 3,500 lbs. That is a starting point from which you can determine what travel trailers can safely be towed by a Highlander Hybrid or any vehicle rated to tow 3,500 lbs. There are a number of factors to be considered, including passenger weight, cargo weight and net payload capacity of your vehicle. You'll also need to take into account applicable legal requirements in areas you plan to visit.
The listed capacity of 3,500 lbs. assumes that you have an optional towing package. Unless you are certain your vehicle already has one installed, check with a dealer and have one installed as needed. The capacity limit includes the trailer with all of its cargo and optional equipment. From the trailer manufacturer's perspective, that is called the Gross Trailer Weight Rating. With few exceptions, that means you are looking for a trailer with a GTWR no greater than 3,500 lbs.
The difference between "curb weight" and "Gross Vehicle Weight Rating" of a tow vehicle is its "payload capacity." The Highlander Hybrid has a payload capacity of 1,359 lbs. or 1,427 lbs., depending on model. Payload capacity is the maximum weight you should carry in your vehicle, including passengers, cargo and trailer weight carried by the hitch (called "tongue weight"). Tongue weight, which varies depending on design and how cargo is distributed inside the trailer, can range from 8 percent to 15 percent of the GTWR.
Your options for the Highlander are a Class II hitch, rated to tow a maximum of 3,500 lbs., or a Class III hitch, rated to tow up to 5,000 lbs. If you plan to "push the limit" in terms of trailer size, go with a stronger Class III hitch. Travel trailers generally transfer more than 10 percent of GTWR to the tongue. That means you should select a weight-distributing hitch with sway control added.
Pushing the Limit
The GTWR of a travel trailer includes Cargo Carrying Capacity, or CCC. A trailer with a GTWR of 4,000 lbs. typically has a CCC of about 1,100 lbs. Keep the actual gross trailer weight below 3,500 lbs. by loading less than 600 lbs. of cargo into the trailer. To determine how much additional cargo you can carry in the Highlander, start with a conservative payload capacity of 1,350 lbs. Subtract a combined weight of 350 lbs. for two adults, and a tongue weight of 450 lbs. for the trailer. That leaves 550 pounds for more passengers and/or cargo.
What You Can Tow
Basically, for the Highlander, you must choose from "ultra light" travel trailers that are less than 20 feet long. A family of five that likes to bring along lots of toys will be limited to "pop-up" trailers or very small specialty trailers such as those made by Scamp, T@b and Utilitoy. See the Resource link for specifications and photos of many ultra light models