Don't get involved with the mail, tell old tenant to complete a Change of Address card at the post office and the mail will forward just fine.

Key card Access for my unit  is managed via front desk so I can't just give a key to sneak them in.

Impossible, try couple dozen, plus paintings, plus vices, plus couple tens of thousand in credit card bills.

@Jonathan Roper Ive gotten a credit card not too long ago to start building up credit.

Try to get at least 5 cards, more if you can. 

I own many properties in Baltimore, houses and commercial units - almost everyday I get mailings from people who say they want to buy my real estate - I get post cards, handwritten letters - silly yellow letters with hand drawn pictures of dollars signs and stick figures of houses.We all at the office have a good laugh, we comment on how awful their attempt to get seller's interest in selling their real estate. 

I transferred my credit card debt with 21+% interest into balance transfer credit cards allowing me to pay off my credit card debt without paying interest (that would later negatively effect my credit score, little did I know, due to opening new lines of credit--C'Est la Vie!)  

You can always join a local credit union and apply or get some credit cards. 

So all of a sudden you have a decline in collected rents, your cash-flow sails out the window, you're paying 18% on credit card debt, and with lower equity you literally can't afford to sell the property because you can't afford the debt payoff and the realtor commission, closing costs, etc.  

I was going to say ask them if you can use it and give them a $25 Amex gift card.