My partner and I were super unsure of which hot springs to go to. There are SO many that all seem to do the same 'thing' but have miniscule differences which feel like whichever you choose will be the difference between a good or a bad day. We spent what felt like days researching which hot springs to go to. We wanted affordable, romantic (it was our anniversary), a natural-like feel, aesthetic and we wanted to be able to spend the whole day there to be able to sunbathe and have the peaceful atmosphere of night. We ended up going with Los Lagos- the others were silly expensive (Tabacon & The Springs), sounded too kid-focused (Baldi) or were only for day OR night (eco-termales).
First of all, the pictures don't do it justice. The image of the pool with the bridge is actually more just a swimming pool with a water slide, not really part of the hot-spring experience and rather, just a bonus as the slide is quite fun. The hot springs themselves were beautiful, of varying temperatures (some a bit too hot for me!), with smaller, more private pools and 2 larger ones. We arrived at 2:30 and it was really quiet which we hadn't expected. There was loads of room and lots of explore. The swim up bar was a nice novelty and they have a food menu too. In total there was over 10 pools, which was more than enough. I do have to add that the service at the bar was terrible- they only had one member of staff on which seemed entirely unfair on them and so you had to wait 20 minutes for a drink. We got the Blue Hawaiian and the Volcan Arenal cocktails and they pretty horrible - too strong and artificial tasting.
We got dinner included for $35 p.p including the springs which we thought was a really great price. It came with a drink, starter, main and dessert from a set menu. The food was pretty below average, it felt cheap and tasteless, so in hindsight, I'd recommend just going to the hot springs and ordering food from the bar (wood fired pizzas etc). However, the service was really good and a nice restaurant.
You also get to visit the ant, butterfly and turtle gardens included in the hot spring price which we were really impressed with. I wasn't expecting much but the ant and butterfly houses were particularly interesting and really educational. The Los Lagos grounds are really beautiful.
Overall, if you are unsure on which hot springs and don't want to spend loads, Los Lagos is the one. We were really pleasantly surprised and wish we had had someone to just tell us to go there straight away to save us all the time researching!