Firstly you need to show that you have an understanding of the type of register and structure needed for such emails. By this we mean you need to be very clear who your audience is and how formal your writing should be. What both have in common is that the content you need to include is specifically provided in the question.
In our informal example offering suggestions on things to do but more importantly supporting them with reasons and explanations is key. Although the answer may be informal you are still expected to demonstrate the ability to use a wide range of language. Take a look at this sample paragraph in answer to the informal email question above.
Informal email – sample paragraph
Local places of interest
As for places to visit, well, we’re in a pretty scenic part of the country and there’s been quite a push from the local tourist office to develop trails and paths around the town. In the main, they are not too physically challenging and generally short enough, although there are a couple of steeper, tougher routes. If that’s not your thing there are some historical locations we could take a look at, ranging from old period houses and monasteries to a vineyard.
This is a good response to the original question. It offers a number of suggestions on places of interest and there is a good range of language with some nice vocabulary like scenic and trails and idioms like not your thing, which are all appropriately used. It also has accurate grammar and uses different linking devices : as for, well, in the main, although to produce a very coherent and informative paragraph.