A Wiffle Golf course can essentially be set up anywhere but there are 3 key factors you need to consider when choosing a location: shelter, landscape accessibility. Most of the advice on this site will be geared towards setting up a course in the forest but that's not to say it's the only way to do it. You could apply it to any space, like your back garden or the middle of a city.
Shelter is the first key consideration and there are two main reasons why you should find a sheltered spot for your course. Initially you want some protection from the elements, those little plastic balls don't tend to fly so straight in the wind and it's always good to get away from the rain. By creating a course in the trees you will naturally get some good protection.
The second reason you should seek a sheltered spot is more about the aesthetic and feel of the whole game. For me, Wiffle Golf should be as much about exploring a new world and landscape as it is about chipping a ball into a hole. You really want to add that air of mystery by making the players work a little to find that first tee. A 5-10 minute walk into the trees will add some anticipation and give the feel that you are hidden away from the rest of the world.
The second key consideration is the landscape and ground covering. You may well have an area in mind, so you should have a think about its suitability. You should be able to create a course without any major contruction work or cutting down trees. Each hole needs to be about 10-15 meters in length but there's no rules on how many holes you should have so think about how many you could fit into your space. Are there any major inclines or declines in the landscape? Setting up a hole on these can add an interesting element but you need to be mindful that players need to get to the green from the hole and pick up any stray balls. A few dug out steps or some rope could help people get around though. You don't need a perfect forest floor between the tee and the hole but players will need to be able to walk between the two and you definitely don't want to be fighting your way through knee high nettles to find a tiny plastic ball, so clearing out any harmful or dense foliage is definitely a good idea. You want to utilise the features of the land while having as little impact as possible, try not to disturb any animal habitats or do too much clearing. Look for flat areas of ground for the tees and greens so you don't have to spend time levelling them off and try to use the features of the landscape to your advantage.
Access is the final key consideration and you'll need to think about it from the the perspective of players and the people building the course. As I've said having a secluded course is great for that air of adventure but if people need to hike for 3 hours with a golf club they're probably not going to bother. Ideally you need a spot that feels isolated but can be accessed easily. You don't need a whole load of materials to create a course but you will need to transport everything there. Pallets for the tees do weigh a few kilos so have a think about how you're going to get all your materials in and out of the course.
One extra element to think about is the flow of your course from the start to the finish. What route will the players take and will your holes have a variety of lengths and difficulties? People will probably want to end their game somewhere relatively close to where they began in order to understand where they are and pick up bikes or cars. If you do manage to make your course on a loop where the start and finish meet, players can keep their game going by playing another round. Signage is also another consideration that can really help with the flow. Ideally each tee should have a number next to it and there should be a little signage to navigate any tricky spots and access points.