#1 money
#2 money
#3 money
For people going travelling, the most common and best advice is, bring fewer things and more money.
Otherwise, they will each want a good backpack and comfortable walking shoes, but both are difficult to buy for somebody else as it's more comfortable to have a good "fit" with both items. Other than that, they will probably want to bring things that are lightweight and not valuable looking (ie not attractive to thieves). They will each want a good, lightweight rain jacket but may well already have one.
Travel guides were useful when I went backpacking many a few years ago, but that was before the world wide web - a good data roaming package might be more useful nowadays, unless they are going places where there is no connectivity in which case, lightweight travel guides = useful.
A few other things I found very useful, were:
--supplies to do laundry in a youth hostel or hotel: travel clothes washing soap, sink stopper, string to hang things to dry afterwards.
--compact travel pharmacy (e.g. pain reliever, meds to deal with upset stomach, plasters & disinfectant, that sort of thing)
--sunscreen & sun hat
--wet wipes
--travel cutlery & food dish & thermos
--bath towel (microfiber is less nice to use, but it dries quickly and takes less space to carry)