I've done the drive from Warsaw, Poland to Tallinn and back this last summer, as well as some shorter road trips in those countries. You've gotten a lot of good advice already, so I'll just add a couple things. In Estonia, Parnu is a nice, quaint beach town on the road around half way from Tallin to Riga. They have some very nice vacation rentals there, at reasonable rates. The beach there is lovely, though quite cold for swimming if it isn't peak season.
Do keep in mind that driving is a little different than you may be used to, lots of slow two lane roads (with lots of speed cameras!) and also an approach to passing with oncoming traffic that is a bit more aggresive than most Americans are used to. It's nothing that you can't handle, jsut expect to have to be more attentive than usual when driving there.
Thanks for the info. Pärnu is on our schedule, from there we will visit Kihnu Island for a day (the island where women have the upper hand). We intend to never drive more than a few hours and we are used to more aggressive driving (Turkey, Greece and Italy were all quite an experience). Our plan is now as follows: Tallinn, Haapsalu, Pärnu, Cesis, Riga, Kuldiga, Klaipeda, Kaunas and Vilnius. We will stay in each of them for a few nights and also explore the surroundings. Lots to see and do. Hope you enjoyed your trip!