There are a few place in town below Rancho Naturalista which would then make a day visit(s) relatively inexpensive.
We stayed in Arenal Obersvatory Lodge this time very convenient and access before and after day trippers but last time we stayed in hotel in La Fortuna - and got shared ride to Arenal; all the lodges round Arenal were quite pricy.
Agree with Andy Savegre Hotel and Spa is great but somewhere like Miriam's diner with cottages across the road is much cheaper and close to Quetzal roadside viewing areas.
Might be worth day trip over to Nicoya Peninsula if you are going past Puntarenas, on way to Cerro / Carara i.e. get early ferry across, shared taxi / bus to Curu (20 mins) then late ferry back - for some more dry forest species, seabirds and cetaceans.
Our recent report might help with what was about in each area if not with accommodation as we tended to go for best available.