Lived and worked in Nicaragua for almost 10 years. Wonderful country, wonderful people, used to be the safest country in Central American, maybe LatAm. SJDS is kinda boring to me, but nice nonetheless.
There is a travel group on the facebooks that deal with the travel to and from Nic and especially the CR/NIC border crossing.
Since the major political upheaval in 2018 and all the sour and bad political stuff, Nicaragua is a fraction of the fun it used to be. The border at Penas Blancas is going to be a PIA, it was never good before the 2018 mess and now that Ortega has pissed everyone off again it is more of a PIA. It is un-predictable in the sense that it might take you an hour to cross or it might take you 5. You could always piss a NIC immigration person off and they won't let you in. Getting to Penas Blancas from Liberia isn't to bad, but you will pay the Gringo tax in terms of cost. Whatever you do, get to the border early as possible.
SJDS can be fun, but you might want to venture out some to some of the other nearby beaches and such. I would always get bored there. If you are going in the fall it is some of the nicest weather that there is, but before about November expect some serious rain.
Nicaragua has really decayed as a tourist spot and the cops aren't cool like they used to and the crime has gone way up according to friends still living there. But overall the people are great and it is interesting to see and check out.