Keeping hydrated is perhaps even more important than having enough food. Being dehydrated can slow you down badly, as well as giving rise to various health issues.
Plain water is the obvious choice during the day (taking care to ensure it's purified as needed). Some folk like to use a 50:50 mixture of fruit juice and water, you can also get flavoured powders.
For hot drinks, there's the obvious candidates of tea, coffee and hot chocolate. We're all familiar with tea bags, instant coffee and instant hot chocolate, but what if, like me, you can't stand instant coffee and want the real stuff!
One solution is Lyons Coffee Bags, available in some supermarkets. The flavour is not bad, and they are convenient to use: just like a tea bag really.
Another approach is a cafetiere of some description, so you can use the same ground coffee that you do at home. Obviously taking a glass and metal cafetiere in your pack is not very practical, but the Smart Café Hot Cafetiere Mug is an ingenious solution which combines cafetiere and mug in one: more convenient than a one-portion plastic cafetiere.