I'm planning 10 days in El Salvador, then 6/7 Days in Guatemala. Can i do a day trip to Nicaragua, Belize, Honduras, Mexico?

Okay, i have a lot of questions and need some help cause im not too experienced with travelling and don't really know where to look to answer my questions and figured some people here might be able to guide me and give suggestions. I'll try my best to make this an more organized post lol.

28F, white, solo travelling (will be 29 once this trip arrives). I'll be going in end of August, beginning of September and I'm not giving myself longer than 15-16 days because... work, boooo lol. But i want to get as many countries in as i can since i'll be in the area. I know i cant go to ALL of these countries in just 15 days, and i want to actually soak up the places and not leave feeling like i barely saw anything cause i was so rushed and it all passed me by, so i recognize i may not end up going to more than just El Salvador and Guatemala, but i figured i'd ask if anyone thinks a day trip or 2 to the surrounding countries would be doable or worth it - and if so, which ones and what i should see to make a day trip worth it. I have a lot of passion about travelling and love solo travelling so lay it on me!!

So i'm starting in El Salvador with friends and some of them are from there (so while i may go off on my own for a day or 2, i'll otherwise have a great tour guide, so im leaning more on reddit and research for the other countries!), then im gunna go on my own to, I think, Guatemala.

I don't want to just stay in 1 place though, i want to EXPLORE. In El Salvador, i'll definitely be seeing San Salvador. Where else might be worth renting a car for to go north of the country? Is there a boat over to Nicaragua for a day trip? Honduras day trip? As for Guatemala, I've heard of Antigua and Flores to visit, and i noticed Flores is pretty close to Belize, so i'm thinking if I start down south of guatemala then make my way up to flores, i could do some sort of tiny trip to Belize from there, then make my way home or something. I don't ONLY want to see the places EVERYONE goes to, you know? I want to leave the city im staying in and explore other parts of the country that are a near by drive and find some special things and places that haven't lost it's charm because of tourists.

I want to see that big volcano but don't know where it is, ruins, beautiful waterfalls, forests, and hikes.

Is Guatemala the kind of place to fly from place to place in or are there busses i could take from Antigua to Flores or literally anywhere or something? Would a hostel be safe in these areas? What suggestions would anyone have for this timeline im going for? What places should i avoid altogether to stay safe?

Anyway, i think this gets my intention across. I know i cant see all of the places but maybe a few day trips to the country over is more doable than i imagine. Just looking for all of the possibilities to choose from!

Thanks in advance :)