One of the biggest dilemmas of those who go on a trip: what to pack? T-shirts, shirts, pants, jeans, socks, underwear, shoes, beauty case, and once moleskine, pens, first aid kit, sunglasses, sarongs ... the list is long and varied.Everyone has different needs and each one has different priorities which influence the choices about what will fill our luggage, so there are no absolute rules and objects really needed. Here then you will find only the guidelines dictated by personal experience, you will have to adapt it to their needs.
Travel costs energy in whatever way it is done. More luggage you have with them and the energy expenditure will be higher. With my experience as bike traveller I can say that you can survive with 10-12kg of baggage for a couple of months without suffering. Much depends on the destination: the Scandinavian winter logically requires different clothing than Florida in the summer. A minimum of information about the weather conditions of the location you will visit, allows you to choose the most appropriate clothing. In general however, it is not necessary parts for heavy clothing while is useful to have some light shirt, maybe old that you can get rid once used. Underwear in abundance (is lightweight and takes up little space), a pair of walking shoes, a swimsuit and a light and big towel for every eventuality. Depending on your needs and your taste how to complete the case, maybe with chargers for phone, camera and lenses, trekking shoes, sunglasses, camping stove, sun cream, hat...
The advice is always to avoid unnecessary!
PS: If you have any other advice to give or want to suggest some object that you'll never leave home when they left, this is the right place ...