There are several ways up Cerro San Cristobal to the Parque Metropolitano, but only two don't require undue exertion. The first is the Teleferico , or gondola, which leaves at the end of Av. El Cerro, a 20 minute walk from Los Leones Metro station. The second if the Funicular , or funicular railway, which starts near the popular Bellavista area and which stops at the Zoo halfway up.  

Both lead up to the viewing area of the park near the summit of Cerro San Cristobal and are a lot easier than riding or walking, although both these are optional extras. A return ticket, travelling on both the teleferico and the funicular cost about CLP$2300 (or U$4) and the whole trip can be done in an afternoon, taking advantage of the metro to get you back to your starting point.

One suggestion is to go up on the Teleferico in the early afternoon, see the sights and then travel down via the Funicular, stopping for a few well earned drinks in Bellavista.

Another refreshing option is to try a Mote con Huesillo (Mow Tay con wess ee yo) which is a sweet, ice cold concoction of rehyrdated dried peaches, wheat, and sugar... best to try rather than think too much about it, it's great. At the top of the hill, on the lookout, several shops sell all manner of tourist gear and one in the middle serves the best Mote con huesillo (600 pesos a big glassful, with peach and wheat, or 400 pesos just for the juice).

There is also the Enoteca restaurant but on a much fancier level. 

There are two public swimming pools in the park, the Tupahue and the smaller Antilén, which are open in summer.