Lahemaa, Estonia's largest national park is certainly worth a visit. Only an hour by car from Tallinn and the roads in the park are excellent. It has a lot on offer, so you have to choose if you only go 1 day. I stayed 3 days in Lahemaa and wasn't bored for 1 minute. I saw the lovely coastline with boulders in Käsmu and Altja, spent a pleasant afternoon on a very nice beach in Võsu. I visited 2 manors Palmse and Sagadi, both equally grand. Just pick one. Plenty of wooden houses, but the most beautiful are in Käsmu.
A lot of forest ofcourse with many trails. The Viru bog trail is the best with a viewing tower in the middle. The Käsmu nature and culture trail with the many boulders is also good. Another good one is the bever trail.
Something totally different was the urban-explorer-style visit to the abandoned Soviet submarine base.
And also important I found excellent restaurants.