Poland and Finland help

Doesn't look like a bad plan at all.

(hope to see Northern lights but not sure of the probability in April)

The chance of seeing the lights in April is not zero, but very close to zero.

Is there something I am missing? Other nearby attractions of interest?

As I mentioned in your previous post it would be helpful to know a bit more about what psychology topics mean in practice. But for a student trip a faster pace is not a bad idea so you can add some other place as well. Had you asked this a couple of years ago I would have suggested going to St Petersburg, but at the moment that is not a good idea. Instead you can take the ferry to Stockholm when after Helsinki and add a day or two there.

Is Helsinki too cold in April?

It depends on where you are from and what too cold means to you, but probably not.

Could also spend 6 nights in Helsinki and take a day trip to
Tallinn....budget may be an issue though, if Helsinki is more
expensive. Could also do 5 nights Krakow and 5 nights Helsinki.

Skipping Tallinn is a bad idea. The museum of occupation seems like it can be a great place to visit if you're studying psychology. https://vabamu.ee/en/

Helsinki is definitely more expensive than Tallinn.

I think one day in Tallinn is awfully short.

I've never been to a sauna. I didn't realize it was a full-day sort of experience. I tend to spend the bulk of my sightseeing time in art and 20th-century history museums or pounding the pavement, looking at the architecture, so I'm not the one to suggest experiences to you.

Both Helsinki and Tallinn could potentially be too cold for me in April. They're likely to be noticeably chillier than Rome is in February, for example. There are climate-summary charts with monthly averages in each city's Wikipedia entry. For a far better idea of the range of weather you might experience, go to timeanddate.com and check the most recent five years' worth of day-by-day statistics.

Helsinki weather April 2023

Tallinn weather April 2023

Use the pull down box at the right just above the graph to change the month and year displayed.

I may be "over my skis" here but, given I also don't know if your students are younger undergrads ,and general psychology at this time, or older with more specific interests.
As a US public school educator Finland's schools are the exemplar. I have a colleague who went via a Fulbright to study their system of libraries. It's a very different approach to that in the US so depending if you have any child or adolescent psych majors, visiting a couple schools (or even university) could be an option?
I also thought the way design is approached, including tech, was fascinating- the Design Museum was awesome and not that large.
There is an incredible sanctuary in the middle of a nondescript parking lot somewhere- wooden, simple benches around- the sound of silence and sense of immediate calm was very cool.

Thank you- these students are generally a little older- 22-39 mainly- and are becoming psychology practitioners (so Man's Search for Meaning/Auschwitz is very relevant!).

The thing is- I want to include Helsinki- I've been there and loved it- but I am a little bit at a loss for what to include while there and wondering if time is better allotted to more time in Poland...

I believe Bon voyage! is describing the Kamppi Chapel of Silence.

I've reviewed my sightseeing list for this summer's trip to Helsinki and don't really see anything especially meeting your needs. It would be interesting if there were (but there unfortunately is not) a museum delving into what it was like in Finland before the break-up of the USSR. Finland shared a very long border with the USSR and had to walk a very fine line to avoid angering its much more powerful neighbor. There's a Lenin Museum in Tampere (really too far away for a day trip) that apparently used to be rather gushingly positive; I didn't see it back then. It has now been reworked to be more truthful, as the museum acknowledges.

Tallinn has quite a few Cold-War-related sights that could be a basis for discussing national emotional trauma. There are similar museums and memorials in Riga, Vilnius and several Polish cities.

Over the last two summers I've spent about 12 days in Helsinki. I am a fan. However, I think for the typical tourist, Tallinn has more to offer. It's older, and its 20th-century history is more dramatic.