La Fortuna can be a disappointment if you're looking for anything Costa Rican . It's over run by tourists so the venders are here in droves with everything they can think of to sell . The alumimum volcano restuarant reminded me of Vegas.
It's OK for a base while you explore the natural surroundings...
The entire city / area is, in one way, a big tourist trap.
That is not to say that there isn't anything great to do here, there certainly is. The Volcano is really amazing to see, the waterfall is great, you can do a lot of stuff like zip-lining, rappelling, eat at SODAs, etc. However, you can do all of this stuff at numerous locations throughout Costa Rica for much less than you would pay here.
While Arenal is the biggest and most famous volcano, there are several other volcanoes to see throughout the country, and it will cost a lot less to see them too. The activities are probably the biggest difference. They seem to be at least twice as much in Fortuna as they are elsewhere. For example, we did zip-lining in Monte Verde for like $40 a person (and it was awesome) whereas the quotes we got for doing it in Fortuna were around $90-$100 per person (over twice as much!)
Restaurants are also more expensive here, with some being obvious tourist places where they charge you the same prices that you would pay back home. They don't even try to convey a Costa Rican appearance. The SODAs were also slightly more expensive here than they were in Monte Verde or San Jose as well.
So, while Fortuna is a great place with a lot to see and do, keep in mind that it's easily the most expensive city in the country and if you are on a budget, you can do everything in other (cheaper) locations across the country.
Unique Suggestions: Go on a volcano tour.
Go see the waterfall.
Take in the city center square, especially on a nice cloudless day.
Fun Alternatives: Monte Verde seems to offer almost everything that Fortuna does, but is much cheaper!