I can't comment on the Ritalin except to say experts disagree. Some say it is OK. Others say it stresses the brain.

You have two months to start being active enough to get a sense of your ability to safely do the activities of the field trip. The important things is to learn you own safe level of doing these activities. You need to establish an "I'm in charge of my own movements' attitude so you don't let others push you into situations that are beyond your safe physical abilities.

You might consider getting a helmet if you will need to be scampering across rocks with unstable footing. A bicycle or skate board helmet or mountaineering helmet may be worthwhile.