I'd definitely stay one of the nights (i.e. Thursday) in Vilnius. There's some great places to hang out there and the old town is really nice to wander through (much more so than Riga for me).

Check out Burbulio Vynine - that's a terrific place if you're into wine, but even if you're not I'd recommend it. Really friendly owners and a nice vibe there, full of locals.

Bukowski Bar is also fun but there are tons of other places. If you just wander round the old town I'm sure you'll find half a dozen places you like without even trying.