I did a tour (my first) of the Baltics about 3yrs ago, mostly Latvia. I echo what psmiffy said about it being very flat, wooded and the sometimes long stretches of gravel off the main roads. The gravel stretches were okay with the bike I rented (700cc hybrid with front sus and 35mm tyres) but slow going.
If you want a break from flatness, there's the hilly (think Lincolnshire Wolds hilly) Gauja National Park about 30miles NE of Riga. IIRC they're jokingly called 'the Latvian Alps'. The SE of Estonia is quite hilly also, you could probably connect and do a decent inland route that way via Tartu.
Also the countryside around 50miles west of Riga is similarly quite rolling and plesant, with some sights in the area (a couple of 'outsider art' type museams, pleasant country towns, and 'worlds most northerly vineyard'). Jumala just west of Riga is good getting in/jumping off point for Riga via the train and worth seeing in itself - a gentrified seaside town by day and a bit of a party town at night. It's also on the edge of a huge bog just to the south which is a national park.