During my visit, I went on a food tour, a city tour, a beer tour, hiking through a national park, bobsledding, to a couple museums, to a traditional Latvian sauna, and more.
The tour I went on was a custom tour offered by Live Riga for the WITS conference, but there are plenty of similar city tours offered by a variety of operators throughout Riga.
I would have loved to explore the Latvian National Museum of Art, Musee Art Nouveau, Riga Ghetto and Latvian Holocaust Museum, Laima Chocolate Museum, and take a tour of the Latvian National Opera.
If you are looking to fit several activities into a one-day trip, these are great ones to start with: a ride on the cable car, a visit to Gutmanis Cave, a tour at the Turaida Museum Reserve and Castle, and a stop at the Sigulda Castle.