I'll put this here rather than the even quieter gaming section.

I need to be able to chat with my dad secretly in order to plan a surprise birthday for mum. However as they are both retired and are never apart it is next to impossible to speak with him privately and they have joint email. It doesn't have to be real time as we need to look into things etc. but needs to be quite regular.

My first thought was some sort of online chess, where we can play at the same time if needed, or correspond. I think this would allow us to chat and is 100% not to raise any suspicions in mum. However I'm astonished by the number of crappy looking and paid for chess games out there. The few free ones I've found are pretty naff and or require extra registrations etc.

Can anyone here recommend something; free, pretty and with I guess asynchronous play with a chat function?

Does he have a smartphone or tablet? Something like Words with Friends that has a chat section, perhaps?

Lots of games on smart phones have in game messaging.

Also, Viber gives the option to put a pin number on private message. You can't access it without it. You only know you have a message from the notification. It doesn't show up who is from. If you get that, you just assume it's from the pin locked conversation and go into it to look

I remember drawsome used to allow chats (usually just a few words).

You said they have a shared email account, but do they have their own mobile phones?
Is Whatsapp or text message not an option?
Messages can be deleted after they have been read.

Something like Between?

No chess but private messaging just for 2 people

Found a little app called Chess Time that fitted all of the criteria, and many others that didn't. Had to be chess for the maternal disinterest, and a game for the subterfuge. Can't play the app but it keeps it lite.