We went to Costa Rica with TUI - loved Costa Rica and very good value for money for a family. I am not sure it would be good value for money on your own though - the rooms were very large. TUI website is down so can't see but think ours was £6300 for 4 of us 2 weeks AI including lots of activities, transfers, hotel and flights (the flights would have been £3,000 of that). Loads of excursions on offer around 20 different ones including Arenal, Monteverde, Carribean side / sloths, Blue River, Buena Vista, Diamante, Nicaragua was next door. We did Arenal, Carribean side / sloths, Buena Vista (ziplining, horse riding, active volanoes, waterslide through jungle - saw monkeys and toucans there), sloth trip, Blue River, and Diamante - very high zipline and wildlife park. At hotel we saw coati, raccoons, geckos, pelican and on trip saw sloths and hotel included windsurfing, SUP and kayaking which we did. Beach and beach volleyball on site.
Our hotel (RUI Guanacaste) was great for things on site and we loved the food - could get waiter service included every night, lots of activities, surrounded by rainforest, kids loved the 5 raccoons that came to every evening meal with us. Its not a couples hotel at all, its a mix of family and quite lively, party type people. Its not somewhere for a quiet break. All kinds of people were there so someone single wouldn't look out of place. Very safe. Best tours were with Gecko Tours from the beach, a Costa Rican company - was just us on the tour they did in their Mercedes van and they told us about history and their lives too - they are flexible and would do whatever we wanted. Lovely guys, very safe.
You may well get better value booking things separately. I wouldn't recommend driving from where we were by - no signposts, roads in poor conditions, herds of animals would suddenly appear on road, lots of driving in middle of roads, driver drove through what looked like 4 rivers with about 80cms of water. You can just get flights with TUI to Liberia or BA go to San Jose. It depends really on how anxious you would be going it alone - it was relatively safe by South American standards, maybe similar risk to London.
I believe TUI also do a grand tour going round and you can do 4 day tours then stay a week at a resort. We met a couple who had done that and they said it was just 4 of them on the trip and they would highly recommend it. We prefer day on tour, day in resort but different people have different preferences and energy levels. The trips were quite full-on starting at 6am or so lasting until dinner time full of activities. So I was happy to have a lazier day next day just kayaking in oceon etc.
If doing it by yourself check Tripadviser/Booking.com - maybe stick to places that are very highly rated and read all reviews. If I was going with kids again I would go back to where we were as kids loved it (both my kids prefer AI and big resorts and 1 ASD/restricted diet, 1 veggie) and easy and safe plus I don't drive and DH wants a break from driving when we are away. Without kids I maybe would do independently with transfers and trips though that's with DH. Without DH would do a tour but I'm not the bravest of people but would be worrying on my own which would ruin some of it.