I spent a lot of time in the Baltic countries a few years ago. They are great destinations and very easy for English-speakers to visit because English is extremely widely spoken (as it is in Poland). There's considerable commonality of recent history (and very good, fully-English-accessible museums to explain it), but the capital cities are quite different architecturally. Much of central Tallinn is medieval; Riga is known for its huge number of Art Nouveau buildings; Vilnius is Baroque. I ended up spending at least a week in each capital city because there was a lot of interest to me as a Cold War junkie. Vilnius appears to get fewer foreign tourists than Tallinn (probably the most touristy of the three) and Riga.
Each country also has interesting secondary cities. In Lithuania I enjoyed Kaunas (especially) and Klaipeda. I used public transportation to get around; the buses are comfortable.
Ground transportation between Poland and Lithuania is challenging, to be sure. Kaliningrad (Russia) and Belarus are sort of in the way. I traveled from Vilnius to Bialystok en route to Warsaw. Bialystok isn't too often on the itinerary of western travelers, but I found it interesting and especially liked the new museum about expulsion (and in some cases voluntary emigration) of Poles to Siberia over the course of centuries.
With limited time I'd look for a flight from Krakow to one of the Baltic capitals. You can't go wrong with any of them. AirBaltic flies to Vilnius, RyanAir flies to Riga and WizzAir flies to Tallinn. Those flights don't necessarily run every day; you can check skyscanner.com for schedules to see what your actual options are. Sometimes available flights depart at really unpleasant (to me) hours like 6:30 AM.
For checking weather I recommend the website timeanddate.com, which provides actual, historical, day-by-day weather statistics for most major destinations. Here's a link for Vilnius:
Vilnius weather -- April 2025
Use the pull-down box at the right, just above the graph, to see other months and years. I like to check data for the most recent five years, because weather (especially shoulder-season weather) can vary a lot. Use the Search box at the upper right to check statistics for other destinations.
Monthly-average precipitation data can be found in the climate-summary charts in the Wikipedia entries for your potential destination cities. Those charts can also be useful for comparing likely temperature ranges in two different months or at two different destinations in the same month. That's sometimes a starting point for me, but my final decision as to whether I'm willing to risk Destination A in Month X comes after I check the temperature charts on timeanddate.com.
April would be too early for me in that part of Europe, and October too late; however, I really dislike chilly weather and have an extremely strong preference for traveling when the days are longer.
I'd recommend getting a guidebook to the Baltic countries. Amazon has books from several major publishers: DK Eyewitness, Rough, Insight and Lonely Planet. There have been a lot of recent complaints about the newest issues in the Lonely Planet series due to an excess of fluff and lack of specificity, but I haven't used the LP guide to the Baltic countries.