Don't think there's anything that will do this for you in the current API.
At network level there's https://developer.cisco.com/meraki/api-v1/#!get-network-traffic which will let you choose 'wireless' but not down to per-AP or per-SSID level. It's all traffic, not just websites, you'd need to process the data to choose just the bits you want and build a table to get the top five.
You can also get data for a list of one or more clients, but that's likely to get messy/hard to scale with large numbers of clients. https://developer.cisco.com/meraki/api-v1/#!get-network-clients-application-usage It won't tell you AP/SSID, you could partition IP space into different ranges for each AP and SSID as a workaround, or use other calls to determine which one a client was using, messy.
Using netflow is another approach, you'd need to set up a collector to take the data and do the analysis, I think you'll still need to use client IP to figure out the AP/SSID.
Alternate approach: on the network-wide > traffic analytics page you can choose wireless, duration and SSID, I think you can deep-link the CSV download. So build a table of deeplinks by network+duration+SSID then iterate through that periodically. Won't give you per-AP though. This probably gets you closest with least effort.
