Simple but effective ciphers

Question

I enjoy cryptography, and like encrypting stuff hands on, for fun. I often send encrypted messages to friends. Trouble is, the easiest ciphers are also very insecure (for example, the Vigénère cipher and the Playfair cipher) and the most secure ones are plain impossible to use with just pen and paper (for example, RSA and AES)! I once even tried to encrypt a message using RSA (using just a calculator, obviously), but, even with some assistance, I gave up halfway through (my message was something like "Hello there!").
Could someoneone suggest a cipher that strikes a reasonable balance between security and feasibility? I just want to have fun, so it's very important that I must be able to do the required calculations by hand, or at most, with the help of a calculator. I have the VIC cipher and a personally modified version of the ADFGVX cipher on mind, but I look forward to suggestions from more experienced cryptographers. P.S.: I was not sure what tags to add, so I hope my choice is relevant. My choice would have been "Recreational Cryptography"!