Estonia was where I did my first bike trip. In July 2006 I rode from my place in Viimsi to Paldiski with a couple of friends from Australia. We attempted to follow the number 1 cycling route but had no map and found signage a bit lacking, so we ended up riding on the uncomfortably narrow and busy highway for a good part of the trip. Since then signing has improved and there are more maps and information available.
Other trips I’ve done with Millerine are Rakvere to Viimsi and Tartu to Viljandi.
In general, Estonia is a great place for cycling in summer. There are quiet roads, regular villages and towns, and most importantly it is very flat. The highest point in Estonia (and all of the Baltics) is Suur Munamägi at a mear 318 metres high.
There are about a dozen official sign posted bike routes throughout Estonia. The number 1 bike route is part of the number 10 Euro Velo route which circuits the nordic countries.
The Esto Velo website contains a good amount of info in English. I purchased rough maps of the bike routes from a bookshop and they have come in handy when a sign post had gone astray. These days the routes are clearly marked on the Open Street Maps Cycle Map, which can be downloaded with a bunch of different apps on smartphones.