If you're looking for a taste of the great outdoors, but you want to manage it on a day trip, and don't really like super difficult trails, Horton Plains is the place for you. Horton Plains is a national park located about an hour outside of Nuwara Eliya. The drive there can be a little steep, and some of the roads aren't in perfect condition. Hotels in Nuware Eliya do offer "tours" of the park, however this could consist of a driver taking you to the entrance and letting you walk the trails on your own. Basically it's nice to get a drive there, but you don't need a guide.
There is a small fee to get into the park, but judging by the good condition of the trails and facilities the money is likely going to the right places. It's a good idea to get to the park as early as possible, as the heat of the day can be pretty serious, and the local wildlife is much more active in the mornings. The local wildlife consists mostly of birds, but there are a few Samba deer around as well. There are some wild cats in the park but tourists rarely spot them.
The big draw of Horton Plains is a spot called "At World's End". It's a giant outlook that looks out over a few other hills and into parts of Sri Lanka. From this spot it seems as though you can see the entire country. This spot is also often the busiest spot as everybody wants to stop and get photos, but if you're patient you can relax and really soak it in. There is also a secondary "At World's End" that's pretty impressive as well.