I configured Two Router on a Stick sites connected by a GRE tunnel.

This took more time and effort that I thought, especially considering that I did not configure such services as DHCP, SSH access, DNS, access lists, QoS or encryption. Perhaps I would have saved some time by using OSPF across the service provider but that would have been less fun. Instead OSPF and EIGRP were redistributed into each other at ISP1. Check out https://networklessons.com/cisco/ccie-enterprise-infrastructure/redistribution-between-eigrp-and-ospf for more info on redistribution.

The configuration was done on a Cisco Modelling Labs instance of Cisco Devnet Sandbox which allows you to run a maximum of 20 nodes concurrently and for free. Quite fun.

With Site1 and Site2 reaching each other over area 0 and AS100, a GRE tunnel is created between G0/0 on both Site1 and Site2. EIGRP AS 101 is created across the point-to-point network on this tunnel and this advsertises the networks 192.168.1.0/24 and 192.168.2.0/24 to each other.

Then, with Router on a Stick config on both sides, the users are able to ping each other.

It is worth noting that EIGRP is rarely used to route traffic to a service provider, OSPF is more preffered because it is an open standard.