Hi! I live in Downtown Baltimore (a few minutes walk to the Inner Harbor or Mount Vernon depending on which direction I head).
Do not rent a car. A lot of the downtown/Inner Harbor/Fells Point/ Harbor East/ Federal Hill area and Mount Vernon (the cultural/historic/artsy area) is walkable. There is also the free shuttle called the Charm City Circulator (CCC) that runs in many directions.
http://www.charmcitycirculator.com/Parking can be expensive- often $6+ for the first 15minutes-1hour. Avoid taxis, they tend to be ruder and overcharge.
I recommend Uber for Bmore and DC. They will pick you up and drop you off where you need to go like a taxi, you use a smartphone to order one and pay so you won't need cash. They have different levels of cars (I use Uber X which costs $5 - $9 for a ride typically- it bases price off distance, etc. Some Uber Levels have a $25 minimum or so because fancier cars are driving you around). I can give you a promotional code if you have never used it before. It would give you a free ride up to $20 for your first ride (and then I would get a free ride for referring you).
The MTA lightrail runs North to South/ South to North from BWI to Hunt Valley and is $1.80 I believe for a one way pass. It's less than $4 for an all days pass or so.
The subway runs East to West (and West to East). It is dirty but useful. It uses the same ticket you use for a bus or lightrail so its easier to swap from one to another. Buses run all over but in nearly two years I've only paid to ride a bus once since everything is so walkable or I can get to it easily via Lightrail or CCC.
The Marc train runs on the Amtrack rails but is waaaaay cheaper. It runs from Camden Yards in Downtown Baltimore to Union Station in DC and back of course.
If you want to park your car somewhere safe and cheaper I recommend not parking in the touristy areas. I can suggest one in my area that's good. It cost $6 on Saturday and $6 on Sunday. But $21/day Monday-Friday. I am sure other people can recommend parking garages based on where you are staying hotel wise (a lot of hotel garages are $35/day if you are staying downtown).
Let me know if you need a list of restaurants based on price, cuisine or atmosphere. :) Or free things to do in the city!!!