Jelgava is an easy trip southwest of Riga, but it has a history of its own. As Mitau, it was the capital of the Duchy of Courland and Semigallia, a small Baltic power whose ambitions show most clearly in the huge palace on Pils Island.
The city was badly damaged in the Second World War, so do not expect a perfect old town. Jelgava is more scattered than pretty, but the best pieces are strong: Jelgava Palace, the Holy Trinity Church Tower, Academia Petrina, and the riverside walks around the Lielupe and Driksa.
It makes sense as a half-day or full-day from Riga, especially if you like baroque architecture, Latvian history, or winter festivals. The city also pairs naturally with Rundale Palace farther south, though Rundale is its own destination rather than a Jelgava sight.