Depends on how much you are using. Probably just put money in the transit card machine, and a transit card would come out.

If you think you are only going to one museum a day, which involve two bus rides, just put money in a transit card would be enough. As it would only cost $4.

If you think you would do more rides, then maybe a one day pass, it cost $5.75. One day pass in chicago CTA is good as it is actually a 24 hour pass.

One day pass can only buy from Walgreens, CVS, etc, not from train station transit card machines though.

For more fare information:
http://www.transitchicago.com/travel...s/default.aspx