if your doing it without the autopilot try useing the rudder input a little to stay on the center

another thing you might be coming accross cross wind (everytime you land i doubt it, but hell anything is possible)

Gems are good, and now more widely available than ever on the internet. You need to find a reliable source though, and prices vary a LOT. But then, if you don't like the price somebody is offering, you can sell to somebody else. They're also easy to hide. I recommend fire opals because their quality is pretty obvious by looking at them (unlike diamonds), they're rare, and the market isn't likely to be flooded with them if certain monopolies are broken.

There are also municipal bonds, used to pay for things in your local government like bridges, but for me that's not tangible enough. It's not like you own the bridge, and local governments are getting bailed out by national ones all the time.