Best time to go to Lao is November to February.
Rainy season starts in (May/) June and ends in October.
March to May is really hot.
You should only consider these months if you want to hike in the Northern mountain area.
Vice versa, this mountain area is very chilly at night in winter (December to Feb.).
Lao is a poor and developing country. The health system is consequently not great.
The care is better in the cities of Vientiane and maybe Vang Vieng.
For really serious health issues it might be best to get to Bangkok.
But don't worry too much. By vaccinations and protection from mosquitos you should be fine. :-)
Lao is a safe country. The crime rate is relatively low. Of course, there are pickpockets where the tourists go. So don't show your valuables too much around.
Don't get involved in drugs. Penalties are hard.
Moreover in some areas there are still mines from the war. In 99% of all cases this is not an issue for tourists. If you walk independently in the country side, be careful, watch the locals and ask them (if possible).
medium to hard
The infrastructure like roads are not great in Lao. You can get everywhere you want but it takes time and sometimes some effort, patience for sure. examples:
Vientane - Vang Vieng: 156km, 3-4 hours
Vang Vieng - Luang Prabang: 227km, 5 hours
I am honest. I traveled Lao in 2010. But I think my experience from then is still mainly valid:
I consider Lao one of the cheapest countries to travel, even for South East Asian standards.
Back then (without smartphone, coursurfing etc) it was my aim to live on 20USD a day. I made it in Lao. :-)
I cannot imagine to book anything in advance in Lao.
Drinking, partying and tubing in Vang Vieng.
Luang Prabang and surrounding is a must- see. The atmosphere of the town is incredible. The landscape around great.
The is more history and beautiful nature around the country.
Vientiane - Vang Vieng - Luang Prabang - further north/west/east (?) - Vientiane - Pakxe (Vat Phou) - Si Phan Don.
Probably, you will start in Vientiane. There city is nothing spectacular but you can spend 1-2 days there.
Next stop can be Vang Vieng for one day of tubing, if you feel like doing something super touristy.
If you go to Lao, you have to go to Luang Prabang. Temples, monks, colonial buildings, chilled bars, beautiful landscape. It is all there. Of course, it gets more touristy but still the place is great. :-)
There are interesting places to go further, Go West for Mekong tours and the rainforest. Go East for caves (Vieng Xai). I went to Phonsavan for the giant, stony Jars. Interesting but not too great and time- consuming to go there and back. You can go north for the mentioned mountain area.
After going back South to Vientiane again, I loved the place Pakxe and seeing the temple area of Vat Phou on the hills.
Further South close to the Combodian border there are the river islands and the dolphins which are hard to spot.