A little bit of common sense goes a long way. As a tourist, the biggest thing you need to account for is street crime and people trying to rip you off. Keeping your fancy things locked up in the hotel or hostel, dressing the part, and having situational awareness on the street, will help ensure that your trip goes smoothly. Seriously, giving somebody less of a reason to rob you, will make you less of a target. Shorts and t-shirts are appropriate in areas with a hotter climate, but in cities like Medellin and especially Bogota, short and flip flops will automatically mark you as tourist. In Medellin shorts are generally okay for doing activities during the day, but leave them at home when you go out at night. Also, taking a bit of extra caution late at night helps too. Hailing taxis on the street during the day is ok, but late at night after a few drinks, it's better to have one called for you or get one from a taxi stand. Also, empty streets late in the evening are generally a bad idea. If your journey is more than a few blocks and the area is quiet, then just take a taxi after a night of drinking.
As for things to do, what are your interests? There is plenty to do in Medellin and its surrounding areas, but narrowing it down to a few specific fields would us give you more accurate information. In addition to asking on this forum, I suggest that you buy a guide book. Many of the things you are asking about, can be easily ascertained by doing and internet search or looking at a guidebook.
I've never done the trip from Guatape to MDE, so I don't know about service to Rionegro (the town where MDE is). I could tell you to backtrack to Medellin and then take a cab from the northern terminal to the airport, but there may be a more direct way from Guatape to MDE.
I think you'll be pleasantly surprised about Colombia. Colombian's are very accommodating hosts and they truly want to put their best foot forward in terms of people visiting their country. They are quite forgiving when foreigners butcher their language, and they will usually be patient and understanding when they see you fumbling for the right words. Also, hostels are a great place to meet people. You can get a pretty good idea about the area you are in by talking to your fellow hostel mates.
Just use common sense,trust your gut, and all will be well. If something doesn't look right or is too good to be true, then it probably is.
-Safe Travels!
