With shoes it really depends on your feet! Different brands have different ratios of length to width etc so it really does depend on what works for you. Basically you have to try a few pairs on and have a good walk around to see what you think. Also don't be afraid of spending a bit of money, shoes which aren't comfy will make the holiday much more painfull, its worth the expense. Also remember good socks to go with the shoes!
If you're doing some serious walking on rough terrain make sure you get a good pair of boots that cover your ankles, and get them here before you go because sometimes its hard to find good quality boots abroad. If however you're not doing much walking off the beaten track just stick to normal/rugged shoes that you can pick up anywhere. Just buy them as you need if you're in somwhere cheap!
IMO Sandles are for lounging on the beach, not for walking in. So unless you're used to walking a lot in them i'd avoid this option because they'll most likely cut your feet to shreads. If you must use them don't rely on them as you main footware, just for when you want a break from you sweaty shoes and you're just walking short distances etc.
