Best way to create a daily snapshotted read-only db in RDS?
What's wrong with bullet 3? Schedule a Lambda function written in your preferred language to run at midnight that calls the RDS API to perform a snapshot.

http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_CreateSnapshot.html 

You want this part near the bottom:

https://rds.us-east-1.amazonaws.com/
    ?Action=CreateDBSnapshot
    &amp;DBInstanceIdentifier=mydbinstance
    &amp;DBSnapshotIdentifier=mydbsnapshot
    &amp;SignatureMethod=HmacSHA256
    &amp;SignatureVersion=4
    &amp;Version=2013-09-09
    &amp;X-Amz-Algorithm=AWS4-HMAC-SHA256
    &amp;X-Amz-Credential=AKIADQKE4SARGYLE/20140423/us-east-1/rds/aws4_request
    &amp;X-Amz-Date=20140423T161105Z
    &amp;X-Amz-SignedHeaders=content-type;host;user-agent;x-amz-content-sha256;x-amz-date
    &amp;X-Amz-Signature=e9649af6edcfbab4016f04d72e1b7fc16d8734c37477afcf25b3def625484ed2