Multi activity instructors
This is a popular way for people getting started in adventure industry. Often the bigger companies realise that they can struggle to get enough qualified applicants, so instead they are looking for people with potential. They will then train you up to meet the standard required to act as an instructor at their centres. Clearly people who do already have some qualifications may stand a better chance of gaining employment, but even if you have no previous experience, this type of instructing can still be within your reach.
Generally the types of activities associated with multi activity centres include open canoes and kayaks, climbing and abseiling, archery and rifle shooting, as well wide games, obstacle courses and outdoor pursuits as obscure as frisbee golf. Clearly you are not going to be applying form many jobs in the future where you will be boasting about how you are qualified to teach the wacky stuff, but if as part of your training you pick up some canoe or climbing qualifications, then these can certainly help you advance a career in outdoor activity instructing. Another great qualification that you will always get at a multi activity instructor, and should really do off your own back if you are serious about teaching outdoors, is first aid. There are different options, but something like the ITC Certificate in Outdoor First Aid is an HSE approved award.
More information about multi activity career training.
Land based activity instructors
While it is possible to get recognised qualifications in a wide range of terra firma adventure activities, probably the most popular would be related to walking and climbing and come in the form of WGL (Walking Group Leader), CWA or CWLA (Climbing Wall and Climbing Wall Leader Award), SPA (Single Pitch Award) and ML (mountain leader Award). Many of these lead on to more advanced mountaineering courses, allowing you to teach people in progressively more hostile or challenging environments.
More information about climbing/mountaineering career training.
Water based activity instructors
If you are happiest on the water then you should be thinking about canoeing, sailing or even windsurfing awards as the most valuable. The British Canoe Union offer a series of qualifications starting out with UKCC Level 1, while the Royal Yachting Association offer the Dinghy Instructor as their starter qualification. Windsurfing is actually very popular, especially if you are hoping to teach around the coast or even around the Mediterranean.
More information about Paddlesports and Sailing career training.